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We have been able to prepare two new lead halides with the formula DMAPbX3 (where DMA is dimethylammonium and X is Cl− or Br−), which display a 4H-hexagonal perovskite polytype, an unusual crystal structure in hybrid organic–inorganic perovskites. Both compounds experience a first-order phase transition, the chloride at ∼320 K and the bromide at ∼250 K, which involves two cooperative processes: an off-center shift of the lead cations and an order–disorder process of the DMA cations. Additionally, a sharp dielectric transition is associated with this structural transition in both hybrids. Both compounds are semiconductors with band gap values of 3.5 eV (X: Cl−) and 3.0 eV (X: Br−). Also, the LT-phase of the Br− compound displays a broad red light photoluminescence (PL) emission (620 nm) and PLE excitation with the maximum at a soft UV wavelength (360 nm). This broadband emission and large Stokes-shifted PL seem to be related to a self-trapped exciton mechanism. Therefore, the uncommon 4H-hexagonal perovskite is a promising crystal structure for understanding and designing new hybrid materials with broad photoluminescent emission and dielectric properties.

In this work, we study the structural changes of the polar [NH4][Cd(HCOO)3] hybrid perovskite under external hydrostatic pressure. We report a reversible framework rearrangement as a function of pressure characterized by: (i) a gradual modification of one formate ligand, which changes its coordination mode from a bridging syn–anti mode at atmospheric pressure (LP-phase) to a chelating-anti mode at high-pressure (HP-phase) and (ii) a change in the coordination of the Cd2+ cations from six-coordinated (LP-phase) to hepta-coordinated (HP-phase). Very interestingly, this unprecedented framework arrangement displays a large electrical polarization. For instance, the polarization value observed at p = 17.7 GPa is about four times the polarization at atmospheric pressure. Therefore, we report that the external pressure induces a novel framework rearrangement in the polar [NH4][Cd(HCOO)3] hybrid perovskite with enhanced electrical polarization. This structure–property relationship offers new insights for designing novel ferroelectric materials based on pressure-responsive hybrid perovskite materials.

Temperature-dependent Raman scattering and differential scanning calorimetry were applied to the study of the hybrid organic-inorganic azide-perovskite [(CH3)4N][Cd(N3)3], a compound with multiple structural phase transitions as a function of temperature. A significant entropy variation was observed associated to such phase transitions, |∆S| ~ 62.09 J&middot, kg&minus, 1 K&minus, 1, together with both a positive high barocaloric (BC) coefficient |&delta, Tt/&delta, P| ~ 12.39 K kbar&minus, 1 and an inverse barocaloric (BC) coefficient |&delta, P| ~ &minus, 6.52 kbar&minus, 1, features that render this compound interesting for barocaloric applications. As for the obtained Raman spectra, they revealed that molecular vibrations associated to the NC4, N3&ndash, and CH3 molecular groups exhibit clear anomalies during the phase transitions, which include splits and discontinuity in the phonon wavenumber and lifetime. Furthermore, variation of the TMA+ and N3&ndash, modes with temperature revealed that while some modes follow the conventional red shift upon heating, others exhibit an unconventional blue shift, a result which was related to the weakening of the intermolecular interactions between the TMA (tetramethylammonium) cations and the azide ligands and the concomitant strengthening of the intramolecular bondings. Therefore, these studies show that Raman spectroscopy is a powerful tool to gain information about phase transitions, structures and intermolecular interactions between the A-cation and the framework, even in complex hybrid organic-inorganic perovskites with highly disordered phases.

We have prepared two new lead halides with the novel general formula of DMA7Pb4X15 (DMA = [(CH3)2NH2]+ and X = Cl– or Br–) by using an easy route under mild conditions at room temperature. These compounds exhibit an unprecedented crystal structure, are formed by layers of distorted [PbX6] octahedra, which share corners and faces, and contain intercalated DMA cations. Very interestingly, they display dielectric transitions, which are related to a partial order–disorder process of the DMA cations between 160 and 260 K. Additionally, these new layered hybrids exhibit a broadband photoluminiscent emission, which is related to the structural distortions of the [PbX6] octahedra. These findings not only open up large possibilities for future optoelectronic applications of these materials, but they also offer a novel playground for an easy modulation of electrical and optical properties of hybrid organic–inorganic materials. We anticipate that this novel A7Pb4X15 formula can be adequate to tune the family of the ...

Cs23Mn16O23 was synthesized via the azide/nitrate route from mixtures of Mn2O3, CsNO3, and CsN3. This manganese(II/III) mixed-valent oxide, which contains only one Mn3+ besides 15 Mn2+ cations, was studied by single-crystal X-ray diffraction and magnetic susceptibility measurements. Its crystal structure [P1, Z = 1, a = 1114.26(2), b = 1185.53(2), c = 1205.39(2) pm, α = 70.596(1), β = 80.377(2), γ = 83.072(2)°, R1 = 0.033] is based on a honeycomb-like arrangement of cesium atoms, providing the space for two different types of one-dimensional polyoxomanganate anions, illustrated by the syntax Cs23[MnO2]4[Mn3O5]4. Magnetic susceptibility measurements indicate dominant antiferromagnetic intra-chain interactions present already at ambient temperature and long range inter-chain magnetic ordering at 11 K.

In this work, we focus on [(CH3)2NH2]PbI3, a member of the [AmineH]PbI3 series of hybrid organic–inorganic compounds, reporting a very easy mechanosynthesis route for its preparation at room temperature. We report that this [(CH3)2NH2]PbI3 compound with 2H-perovskite structure experiences a first-order transition at ≈250 K from hexagonal symmetry P63/mmc (HT phase) to monoclinic symmetry P21/c (LT phase), which involves two cooperative processes: an off-center shift of the Pb2+ cations and an order–disorder process of the N atoms of the DMA cations. Very interestingly, this compound shows a dielectric anomaly associated with the structural phase transition. Additionally, this compound displays very large values of the dielectric constant at room temperature because of the appearance of a certain conductivity and the activation of extrinsic contributions, as demonstrated by impedance spectroscopy. The large optical band gap displayed by this material (Eg = 2.59 eV) rules out the possibility that the observ...

In this work we prepare and study the M(dca) 2 pyz compounds, where dca = polar dicyanamide N(CN) 2 − ligand, pyz = pyrazine (C 4 H 4 N 2 ) ligand and M = Fe 2+ , Co 2+ and Zn 2+ that, according to DSC display a first order phase transition, at T t ∼ 270 K (Fe), ∼250 K (Co) and ∼240 K (Zn). Very interestingly these compounds, which consist of a rather robust framework with incorporated polar dca ligands, and which does not host guest species in the cavities of the structure, display a dielectric anomaly associated to such phase transition. This latter in fact corresponds to the structural transition from the disordered HT-phase (space group Pnma ) to the ordered LT-phase (space group P 2 1 / n ). From a detailed analysis of the dipole distributions in each of these crystal structures we conclude that the polar dca ligands, which bridge two transition metal cations, display a cooperative electrical arrangement, in which the dipoles of each of the two interpenetrated networks show a ferroelectric (FE) arrangement; nevertheless, the relative antiparallel orientation of both networks finally results in net antiferroelectric (AFE) arrangements both in the LT- and HT-polymorphs. Very interestingly, the dynamic disorder of the dca ligands at T t gives rise to the observed dielectric anomaly. These materials constitute, therefore, an example of hybrid organic–inorganic compounds where the dielectric response directly arises from the framework –and not from polar guest species– which is a very interesting and novel approach in the search for materials in which the dielectric and magnetic properties can couple more strongly.

The perovskite azido compound [(CH3)4N][Mn(N3)3], which undergoes a first-order phase change at Tt=310 K with an associated magnetic bistability, was revisited in the search for additional ferroic orders. The driving force for such structural transition is multifold and involves a peculiar cooperative rotation of the [MnN6] octahedral as well as order/disorder and off-center shifts of the [(CH3)4N]+ cations and bridging azide ligands, which also bend and change their coordination mode. According to DFT calculations the latter two give rise to the appearance of electric dipoles in the low-temperature (LT) polymorph, the polarization of which nevertheless cancels out due to their antiparallel alignment in the crystal. The conversion of this antiferroelectric phase to the paraelectric phase could be responsible for the experimental dielectric anomaly detected at 310 K. Additionally, the structural change involves a ferroelastic phase transition, whereby the LT polymorph exhibits an unusual and anisotropic thermal behavior. Hence, [(CH3)4N][Mn(N3)3] is a singular material in which three ferroic orders coexist even above room temperature. Triple-ferroic hybrid perovskite: The azido perovskite compound [(CH3)4N][Mn(N3)3] is a singular multiferroic material in which the three ferroic orders (electric, elastic, and magnetic) coexist even above room temperature (see figure).

We report the barocaloric performance of cadmium-dicyanamide perovskite [(CH3CH2CH2)4N]Cd[N(CN)2]3 ([TPrA]Cd[dca]3). The introduction of Cd, a large second-row post-transition metal, into this hybrid organic–inorganic perovskite structure leads to three separate thermally driven phase transitions and four polymorphs, which we characterize in full using temperature-dependent X-ray diffraction and temperature-dependent calorimetry. Amongst these transitions, the non-isochoric orthorhombic to tetragonal phase transition at high temperatures, with a large volume change of ∼0.4%, leads to a large reversible pressure-driven isothermal change in entropy of ∼11.5 J K−1 kg−1, and a giant barocaloric tunability of the transition temperature with pressure of ∼38.2 K kbar−1, which are both desirable for future low-pressure barocaloric cooling applications.

We report on the hybrid inorganic-organic ammonium compound [NH4][Cd(HCOO)3], which displays a most unusual framework structure: instead of the expected 4(9)·6(6) topology, it shows an ABX3 perovskite architecture with the peculiarity and uniqueness (among all the up-to-date reported hybrid metal formates) that the Cd ions are connected only by syn-anti formate bridges, instead of anti-anti ones. This change of the coordination mode of the formate ligand is thus another variable that can provide new possibilities for tuning the properties of these versatile functional metal-organic framework materials. The room-temperature crystal structure of [NH4][Cd(HCOO)3] is noncentrosymmetric (S.G.: Pna21) and displays a polar axis. DFT calculations and symmetry mode analysis show that the rather large polarization arising from the off-center shift of the ammonium cations in the cavities (4.33 μC/cm(2)) is partially canceled by the antiparallel polarization coming from the [Cd(HCOO)3](-) framework, thus resulting in a net polarization of 1.35 μC/cm(2). As shown by second harmonic generation studies, this net polarization can be greatly increased by applying pressure (Pmax = 14 GPa), an external stimulus that, in turn, induces the appearance of new structural phases, as confirmed by Raman spectroscopy.

Here we present an in-depth study of the magnetic properties of a family of metal–organic perovskites ABX3, [CH3NH3][M(HCOO)3] in which A = CH3NH3+ is the methylammonium cation, B = M is a divalent metal cation (Mn2+, Co2+, Ni2+ or Cu2+), and X is the formate anion (HCOO−). The magnetic properties have been measured on powdered samples and along the different orientations of mm-sized single crystals. They display spin-canted weak ferromagnetism with Néel temperatures of 8.0 K (Mn2+), 15.7 K (Co2+) and 34 K (Ni2+), which are inversely proportional to the ionic radii of the metal cations. The Cu2+ member displays low-dimensional magnetism as a result of orbital ordering of the Cu2+ d orbitals originating from a Jahn–Teller distortion. Pulsed-field magnetization experiments (fields of up to 60 T at temperatures down to 0.6 K) show that Mn2+, Co2+ and Ni2+ formates display cation-characteristic spin flop transitions. A saturation magnetization value of 5 μB (at 12.5 T) was observed for Mn2+, meanwhile the Co2+ formate shows an orientation dependent quasi saturation (5.1 μB at 21 T along [101] vs. 5.8 μB at 26 T along [010]). The different isotropic/anisotropic behaviour can be explained by the orbital contribution to the magnetic response The Spanish authors are grateful for financial support from Ministerio de Economía y Competitividad (MINECO) (Spain) and EU under the project ENE2014-56237-C4-4-R, and Xunta de Galicia under the project GRC2014/042. L. C. G.-A. acknowledges UDC for a predoctoral fellowship and Fundación Barrié for the research stay grant at LANL. Work at LANL, A. P. H. and B. P.-D.'s visit to LANL were funded by the Laboratory Directed Research and Development program at LANL. The NHMFL pulsed-field facility is funded by the U.S. National Science Foundation through Cooperative Grant No. DMR-1157490, the State of Florida, and the U.S. Department of Energy SI

We have found that the hybrid organic-inorganic perovskite-like formate Mn(HCOO)(3)[(CH(3))(2)NH(2)] shows a dielectric transition around 190 K. According to single crystal X-ray diffraction, the compound shows rhombohedral symmetry at room temperature and monoclinic symmetry at low temperature (100 K), and the main difference between both structures is that the (CH(3))(2)NH(2)(+) (DMA) cations are disordered in the high temperature phase but cooperatively ordered in the low temperature one. The vibrational spectra of this compound reveal that significant changes take place in the vibrations ascribed to the DMA cation (changes in the frequency of certain vibrations, splitting of particular vibrations, and changes in the intensities), while no significant changes have been observed in those attributed to the formate anion. On the basis of all this information, we attribute the origin of the dielectric transition to the dynamics of the DMA cations: above 190 K these cations can rotate inside the cubooctahedral cavity created by the [Mn(HCOO)(3)](-) framework, while for lower temperatures such rotation gets frozen, and their cooperative arrangement inside the cavities give rise to the observed dielectric transition.

Se han conseguido preparar monocristales de 10 oxidos mixtos de la serie LnMn2O5 (Ln= Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho y Er) mediante sintesis hidrotermal optimizada, en un unico paso y sin tratamientos termicos posteriores partiendo de las correspondientes sales metalicas en disolucion acuosa a 240 oC. Los monocristales obtenidos son relativamente grandes, de varias micras y su morfologia varia a lo largo de la serie: es poligonal en el caso de los compuestos de los lantanidos del inicio de la serie (Ln= Pr, Nd, Sm, Eu y Gd) y acicular en el caso de los compuestos de Y y de los lantanidos del final de la serie (Ln= Tb, Dy, Ho y Er). Tras el analisis de los productos obtenidos empleando distintas condiciones de sintesis atribuimos la diferente morfologia a una mayor velocidad de crecimiento cristalino a lo largo del eje c cuando intervienen los iones mas pequenos (Y, Tb, Dy, Ho y Er) debido a la mejor adaptacion de estos ultimos a la estructura cristalina del compuesto.

In this work we introduce the use of a combined Rietveld refinement and Maximum Entropy Method (MEM), as a helpful new tool to gain insight into the charge ordering pattern present in CO-transition metal compounds. In particular we show the study made on two CO oxyborates A2OBO3 (A = Mn and Fe): (a) the Mn-oxyborate, that exhibits a strong charge lattice coupling, and whose charge ordering pattern had been previously satisfactory established from BVS analysis of single crystal X-ray diffraction data and that we have used as a test to confirm the validity of the new method; (b) the weakly charge-lattice coupled Fe-oxyborate, a compound not suitable for the BVS approach and for which nevertheless this method gives evidence for a new charge ordering pattern.

A multistimuli response to temperature and pressure is found in the hybrid inorganic–organic perovskite-like [TPrA][Mn(dca)3] compound, which is related to a first-order structural phase transition near room temperature, Tt ≈ 330 K. This phase transition involves a transformation from room temperature polymorph I, with the noncentrosymmetric space group P4̅21c, to the high temperature polymorph II, with the centrosymmetric space group I4/mcm, and it implies ionic displacements, order–disorder phenomena, and a large and anisotropic thermal expansion (specially along the c-axis). As a consequence, [TPrA][Mn(dca)3] exhibits a dielectric anomaly, associated with the change from a cooperative to a noncooperative electric behavior (antiferroelectric (AFE)–paraelectric (PE) transition). The former implies an AFE distribution of electric dipoles in polymorph I, related to the described off-shift of the apolar TPrA cations and the order–disorder of the polar dca ligands mechanisms, that are different from those reported, up to now, for others perovskite-type hybrid compounds. Such cooperative electric order, below Tt ≈ 330 K, coexisting with long-range antiferromagnetic ordering below T = 2.1 K render the [TPrA][Mn(dca)3] a new type-I multiferroic material. In addition, the obtained experimental results reveal that this compound is also a multistimuli-responsive material, with a very large sensitivity toward temperature and applied external pressure, δTt/δP ≈ 24 K kbar–1, even for small values of pressure (P < 2 kbar). Therefore, this material opens up a potential interest for future technological applications, such as temperature/pressure sensing., The authors are grateful for the financial support from Ministerio de Economia y Competitividad MINECO (MINECO) ENE2014-56237-C4-4-R and Xunta de Galicia under the project GRC2014/042. J. M. B.-G. also wants to thank Barrié Foundation for a predoctoral fellowship and S. Y.-V. to the Xunta de Galicia for a postdoctoral grant (Plan I2C).

After half a century of intensive and extensive studies on ferrocene [Fe(C5H5)2] and ammonia-borane (H3N·BH3), it is very exciting to observe that these “classical” compounds still hide interesting properties never described before. Despite it being well-known that they both experience phase transitions as a function of temperature, here, for the first time, we give experimental evidence for the dielectric transitions they experience associated to the former, results that we support with DFT calculations. In that context, we report that ferrocene displays a temperature-induced paraelectric to antiferroelectric transition associated with the monoclinic–triclinic transition, which implies order–disorder of the cyclopentadienyl (Cp) ligands and displacement of the Fe atoms within the ferrocene molecules. As for the ammonia-borane, we report a sharp dielectric transition at T ≈ 232 K, associated with a structural transition that combines ordering and atomic displacement of the H3N·BH3 molecules. In this case, we attribute such behaviour to the temperature dependent displacement of the H3N·BH3 molecules out-of the crystal polar c-axis The authors are grateful for financial support from Ministerio de Economía y Competitividad (MINECO) ENE2014-56237-C4-4-R and Xunta de Galicia under the project GRC2014/042. S. Y.-V. acknowledges Xunta de Galicia for a Postdoctoral fellowship SI

By using the nitrate decomposition method in the presence of KNO 3 , we have been able to enlarge the compositional range of the cobalt-layered compounds Ln 1-x Sr 1+x CoO 4 (Ln: La, Nd, Gd) to 0≤x≤0.40 in the case of lanthanum, to 0≤x≤0.30 for neodymium, and to 0≤x≤0.20 in the case of gadolinium. In this work we present the results of their structural characterization by means of powder X-ray diffraction (PXD) and Rietveld analysis as a function of composition (x) and the rare earth (Ln), information that has been complemented by infrared spectroscopy (IR), electron diffraction (ED) and electron microscopy.

By using the nitrate decomposition method in the presence of KNO3, we have been able to enlarge the compositional range of the solid solution of the layered compounds with K2NiF4 structure Ln1−x Sr1+xCoO4 (Ln: La, Nd, Gd) to 0 x 0.40 in the case of Ln = La, to 0 x 0.30 for Ln = Nd and to 0 x 0.20 for Ln = Gd. In this paper we present the results of their structural characterization by means of powder X-ray diffraction and Rietveld analysis as a function of composition (x) and the rare earth (Ln), information that has been complemented by the results obtained from electron diffraction and transmission electron microscopy.  2003 Elsevier SAS. All rights reserved.

We report the synthesis of the Ruddlesden–Popper material Sr 3 Fe 1.5 Co 0.5 O 6.67 (a compound usually obtained under high-pressure conditions) by a simpler normal pressure liquid-mix method and its structural characterization by means of X-ray powder diffraction. In this material—which is semiconducting and shows a glassy magnetic behavior—the combination of the reduced dimensionality of the structure compared to 3D perovskites, the small grain size obtained by our synthesis method and the selected stoichiometry, results in a record magnetoresistance in these kind of compounds, −47% at T =5 K and H =50 kOe, that is explained on the basis of intragranular and intergranular effects.

HREM, EDX and EELS techniques have been used to study the transformations that occur in the high-temperature superconductors YBa 2 Cu 3 O 7 − δ (commonly referred to as ‘123’) and YBa 2 Cu 4 O 8 (‘124’) in their reaction with the lithiating agent n-BuLi. This study confirms that the original ‘123’ and ‘124’ materials transform into a heterogeneous product that mainly consists of higher members of the Y 2 Ba 4 Cu 6 + n O 14 + n homologous series of compounds. The amount of lithium present in these new phases is quantified through systematic EELS experiments. Study of the energy-loss near-edge structures (ELNES) of the Li-K edge shows that Li is present in the main phases of the lithiated YBaCuO materials under two different forms: (i) in an ionic state (ii) in a more metallic-like state. These results allow to rationalize both the high lithium ionic conductivity displayed by these materials and the influence of lithium on the superconducting properties of these cuprates.

The title compound is synthesized by the azide/nitrate route from Mn2O3, CsNO3, and CsN3 (closed steel vessel, 773 K, 30 h) and characterized by powder and single crystal XRD, magnetic susceptibility measurements, and DFT calculations.

We report that the hybrid organic-inorganic compound [(CH(3))(2)NH(2)][Mg(HCOO)(3)] shows a marked dielectric transition around T(t) ∼ 270 K, associated to a structural phase transition from SG R3[combining macron]c (centrosymmetric) to Cc (non-centrosymmetric). This is the highest T(t) reported so far for a perovskite-like formate that is thus a promising candidate to display electric order very close to room temperature.

HREM has been used to study the process induced by the reaction of the HTSC YBa{sub 2}Cu{sub 3}O{sub 7{minus}y} (commonly referred to as '123') with the lithiating agent n-BuLi. High resolution electron micrographs reveal that this reaction promotes, at room temperature, irreversible microstructural modifications into the '123' structure leading to a rather heterogeneous product. In this respect, both '247' (Y{sub 2}Ba{sub 4}Cu{sub 7}O{sub 15{minus}z}) and the '124' (YBa{sub 2}Cu{sub 4}O{sub 8}) structures appear as either extended defects within the '123' matrix or disordered intergrowths. A structural model is proposed to justify the high lithium conductivity displayed by these materials.

Following our work on lithium insertion into YBa 2 Cu 3 O 7− δ (commonly referred to as “123”) in which we observed the room-temperature formation of YBa 2 Cu 4 O 8 (“124”) [1,2], we have now found, by the same procedure using “124” as starting material, a novel phase with Y:Cu:Ba in the ratio 1:2:6 (“126”), intergrown in different proportions with YBa 2 Cu 4 O 8 and YBa 2 Cu 5 O 9 (“125”). Powder X-ray diffraction patterns and electron microscopy and diffraction suggest a unit cell of ≈ 3.80 × 3.86 × 33.4 A containing four [CuO 4 ] chain layers instead of the two observed in the original “124”.

We report the structure and microstructure of the Ruddlesden-Popper (RP) series n=2 Gd2−2xSr1+2xCo2O7 (x = 0 and 0.10). X-ray powder diffraction, electron diffraction and HRTEM indicate that these compounds have a tetragonal structure (space group: P42/mnm) with cell parameters a ≈ 5.3749A c ≈ 19.3591A, in which the [CoO6] octahedra are distorted and tilted. Also, very interestingly, according to the obtained results these compounds presents A-site cation ordering where the divalent and bigger Sr2+ cation selectively prefers the 12-coordinated P site while the smaller and trivalent Gd3+ cation prefers the 9-coordinated R site. From the electrical point of view these materials are semiconducting and their resistivitiy decreases upon doping. In addition, the highest doped sample presents a small magnetoresistive effect MRmax = −3.2 % under Hmax = 50 kOe at T = 55 K.

This is the accepted manuscript of the following article: Señarís-Rodríguez, M. et al. (1999). Peculiarities in the electrical and magnetic properties of cobalt perovskites Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+). International Journal Of Inorganic Materials, 1(3-4), 281-287. doi: 10.1016/s1466-6049(99)00042-2 We refer here to the electrical and magnetic properties of the Ln1−xMxCoO3 systems (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+), paying special attention to those ferromagnetic compounds that display M–I transitions as temperature rises: La1−xMxCoO3 (M2+: Ca2+, Sr2+, Ba2+) in the compositional interval x=0.2–0.3, and Nd1−xSrxCoO3, with x=0.40. The magneto-transport properties of such materials are peculiar and interesting: they show diodic behavior and large relaxation effects — these latter being specially important in the Nd compound — they display magnetoresistive effects specially at the M–I transition temperatures, and they age with time. All these results are discussed on the basis of the inhomogeneous electronic structure of these doped cobalt perovskites and taking into account the influence of the lanthanide ion on their magnetic and electrical properties We thank the Spanish DGICYT for financial support under project MAT98-0416-C03-02 SI