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Local dopant compositions within individual α-Sialon grains were measured by analytical electron microscopy (AEM) in hot-pressed CaxSi12−3xAl3xOxN16−x (x = 0.3–1.4) ceramics. The reduction of local x values from the nominal dopant compositions is about 40% in general, and it reaches 60% for the end member (x = 1.4) which contains inclusions of AlN-based 21R phase. This results exhibit stronger departures from x than the previous report of 30% dopants missing in α-Sialon phase by electron probe micro-analysis (EPMA) [J Eur Ceram Soc 19:1637, 1999]. Amorphous films of ~1 nm thick were commonly found at grain boundaries (GBs), which could only take a small fraction of undetected dopants while the film composition exhibits a quite different behavior. The general presence of GB films can rationalize the discrepancy between AEM and EPMA results by their differences in probe size and detection geometry, while the much larger gap in the end member suggests the existence of Ca-rich glasses in the intergranular regions. By excluding this end member, a linear relation between dopant solution and lattice expansion is restored in α-Sialon structure, which leads to 20 and 80% increases of the expansion coefficients from those given in the previous and original reports, respectively. This study not only demonstrated the necessity of solubility study in ceramics by AEM refinement, but also opens a new front to correlate the solution behavior with the intergranular glass/amorphous structures, both were regarded so far as largely independent.

The influence of the spark plasma sintering (SPS) heating rate on densification, microstructure and strength of ZrB2–20 vol.% SiC composites with or without 5 vol.% Yb2O3 addition was assessed. A low heating rate promoted densification, induced grain growth and slightly increased the strength in the Yb2O3-free ceramics, whereas the low heating rate inhibited the densification, maintained the grain size and greatly decreased the strength in the Yb2O3-doped composites.

ZrB2–20 vol.% MoSi2 composites with elongated ZrB2 grains were prepared by reactive hot pressing using Zr, Si, B and Mo as raw materials. The ductile behavior of MoSi2 at high temperature helped the rod-like ZrB2 grains align under the applied pressure during sintering, resulting in a partly textured composite. The anisotropic growth of ZrB2 grains improved the toughness of the composite remarkably.

A suspension stabilizer-coating technique was employed to prepare x mol% Yb2O3 or Gd2O3 (x = 1.0, 2.0, 3.0 and 4.0) co-stabilized 1.0 or 2.0 mol% Y2O3 doped ZrO2 nanopowders (xYb1Y, xYb2Y, xGd1Y). Fully dense Yb2O3/Gd2O3 and Y2O3 co-doped ZrO2 ceramics were obtained by means of pressureless sintering in air at 1450 °C. Phase assemblage, microstructure measurements indicated that polycrystalline tetragonal ZrO2 (TZP) was achieved in the 1Yb1–2Y and 1Gd1–2Y grades, whereas the 4Yb1–2Y ceramic was almost fully stabilized cubic ZrO2 (FSZ) and all other investigated stabilizer combinations resulted in partially stabilized zirconia (PSZ). Complex impedance analysis showed that the conductivity of the xGd1Y and xYb1–2Y co-doped zirconia ceramics increased with increasing Gd2O3 and Yb2O3 content. The xGd1Y grade had a lower conductivity than the xYb1Y and xYb2Y ceramics. The conductivity behaviour was assessed and explained in terms of the cubic phase content and dopant ionic radius.

Adding a small amount of vanadium carbide as sintering aids, nearly fully dense ZrB2–SiC composites were obtained by pressureless sintering methods at 2000–2100 °C. Thermodynamic calculations and experimental results proved that VC is better at removing the surface oxide impurities of ZrB2 than WC, and played a very important role during the densification process. The mechanical properties of the sintered ceramics were also reported.

ZrB 2 –20 vol%SiC ceramics without and with additive of 2 wt% carbon nanotubes (CNTs) were fabricated by hot-pressing at 1900 °C for 1 h under 30 MPa using ZrB 2 , SiC and CNTs as starting materials. The fracture toughness increased about 15%, but hardness, flexural strength as well as thermal conductivity did not vary significantly from the addition of CNTs.

The isothermal oxidation behavior of Cr2AlC ceramics oxidized in air at 1,100 and 1,250 °C for 20 h was studied. The phase compositions and microstructure of the oxidized surface were identified and observed by XRD and electron probe microanalysis (EMPA), respectively, while the cross sections of oxidized samples were also examined by EMPA equipped with energy dispersive spectrum capabilities. The results indicated that the oxidation of Cr2AlC samples was carried out by the outward diffusion of Al, together with small amounts of Cr, and the inward diffusion of O to form a surface layer of α-Al2O3, while carbides (Cr7C3 and Cr3C2), rather than oxides (Cr2O3), were formed in a layer under the surface. The mass gain per unit surface area of oxidized Cr2AlC followed a parabolic relation with oxidation time, and the parabolic rates, kp, for oxidation at 1,100 and 1,250 °C were 1.1 × 10−12 and 7.1 × 10−10 kg2 m4 s−1, respectively.

Bi 3.15 Nd 0.85 Ti 3 O 12 ceramic, which is a three-layer ferroelectric Aurivillius phase, was prepared by spark plasma sintering. The effect of texture on the anisotropy of dielectric, ferroelectric and piezoelectric properties was studied. X-ray diffraction showed that samples perpendicular to the hot-pressing direction had a – b -axis preferred texture, whereas, samples parallel to hot-pressing direction had c -axis preferred orientation. The dielectric constant, remanent polarization and piezoelectric constant of samples with orientation close to a -axis are larger than those of samples with orientation close to c -axis. Their Curie points are all about 410 °C.

The investigation of bulk Cr2AlC ceramic fabricated by Spark Plasma Sintering (SPS) from coarse powders (CAC10) and fine powders (NCAC10) in the temperature range of 1100–1400 °C was carried out. The XRD results indicate that Cr2AlC, as major phase, always appears with minor and trace amount of Cr7C3 and Cr2Al respectively in both NCAC10 and CAC10 samples and the amounts of later two phases decrease with increase in temperature. However, the Cr2AlC phase content in NCAC10 is higher than that of CAC10 sintered at the same temperature. The micrographs of back-scattered SEM show that grains with smaller size and pores with fewer amounts appear in SPSed NCAC10 in comparison to that of CAC10. As consequence, the higher hardness (5.6 GPa) of NCAC10 than that (3.9 GPa) of CAC10 was obtained. The patterns of XRD, microstructure and hardness of samples HPed at 1400 °C for the same composition were also presented for comparison. © 2007 Elsevier B.V. All rights reserved.

Four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2–SiC–AlN, were synthesized in situ via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS), using Zr, Si, B4C, BN and Al as raw materials. The synthesis process plays a critical role in the microstructural features of the composites obtained. The R-SPS process can lead to a more homogeneous and finer microstructure due to its high heating rates and short holding time, while the RHP process is likely to result in coarse microstructures due to a long enough holding time for grains growth.

ZrB 2 –SiC–C ultra high temperature ceramics (UHTCs) have been produced by hot pressing pyrolyzed mixtures of ZrB 2 and polycarbosilane (PCS). Samples with SiC contents of 0%, 5% and 16% in volume derived from PCS were prepared. The phase composition, microstructure and mechanical properties were characterized for composites hot pressed at 2073 K for 60 min under the pressure of 20 MPa in an argon atmosphere. Analysis showed that the addition of PCS improved the relative density from 78% (without PCS addition) to ∼ 100% (with 16% SiC derived from PCS addition). Hardness and fracture toughness of the composite were also improved.

The reaction process of Cr 2 AlC ceramics was analyzed, in which the samples were prepared for composition Cr:Al:C = 1:1.2:1 by hot-pressing in argon in the range of 850–1450 °C using Cr, Al and graphite powders as the starting materials. X-ray diffraction (XRD), electron probe microanalysis (EPMA) and energy dispersive spectrum (EDS) were employed for identification of phase assembly and analysis of reaction route of the samples. The phase formation sequence of Cr 2 AlC was finally analyzed based on phase diagram of the Cr–Al binary system combined with the results of differential thermal analysis (DTA) and XRD. It was found that Cr 5 Al 8 , Cr 2 Al and Cr 7 C 3 were the intermediate phases appearing in turn in the heating process. The amount of Cr 2 AlC phase was gradually increased with increase in temperature by the reaction between Cr–Al intermetallic compounds, un-reacted Cr and graphite, and it became a pure phase in the sample with disappearance of intermediate phases above 1250 °C.

Cr2AlC ceramics were fabricated by hot-pressing using Cr, Al and C powders as starting materials. The phase assemblages of the samples consisted of Cr2AlC, as a major crystalline phase, together with a very small amount of Cr7C3 and an unknown phase. Its thermal and electrical as well as mechanical properties were determined.

Effective grain alignment in Bi4Ti3O12 ceramics by superplastic-deformation-induced directional dynamic ripening

The characteristics of Eu-stabilized α-sialon ceramics derived from self-propagating high-temperature synthesis (SHS) Eu α-sialon powders without and with the addition of Y 2 O 3 are investigated. The results showed that the amount of α-sialon phase formed in sintered Eu α-sialon composition was much less than that in SHS-ed powder when the composition was hot-pressed at 1800 °C for 1 h, while the transformation of α-sialon to β-sialon phase did occur at the same time, which could be attributed to the metastability of SHS-ed powder because of the high heating and cooling rate during the SHS process and the reduction of Eu 3+ to Eu 2+ under the reduction conditions during hot pressing. By addition of Y 2 O 3 into SHS-ed Eu α-sialon powder, thus to form (Y,Eu) α-sialon phase in the sintered sample, the stability of α-sialon phase was improved, as the ratio of α-sialon to α-sialon was increased from 70 wt.% in SHS-ed powder to 83 wt.% in the sintered product by 50 mol% of Y 2 O 3 added into SHS-ed powder.

Different ratios of Ce to Y were selected to stabilize (Ce,Y) α-sialon during self-propagating high-temperature synthesis (SHS). The phase assemblages were then studied for both SHS-ed (Ce,Y) α-sialon powders and sintered ceramics. The results showed that the amount of α-sialon phase increased as a main crystalline phase in SHS-ed powders when 20 at.% of Ce was replaced by Y in the composition, and further increase in the substitution of Y for Ce did not obviously increase the α-sialon content, whereas β-sialon was the major phase in the SHS-ed powders without the introduction of Y in the composition. The transformation of α-sialon to β-sialon did occur when the SHS-ed powders were hot-pressed for the compositions without Y and substitution of Y for Ce less than 40 at.%. The 50 at.% replacement of Ce by Y was enough to stabilize (Ce,Y) α-sialon structure as the (Ce,Y) α-sialon phase was kept in the sample with similar amount to that in the SHS-ed powders.

A novel stabilizer coating technique was applied to prepare 1.0 mol% Nd2O3+1.0 mol% Y2O3-co-coated ZrO2 nanopowder from an alcohol/water-based suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia powder. The low stabilizer content starting powder could be fully densified into a (1Nd,1Y)-TZP ceramic when pressureless sintering in air at 1450 8C. The obtained ceramic revealed an excellent fracture toughness of 12.5 MPa m 1/2 in combination with a hardness of 10 GPa, a combination of properties that is comparable with that of the traditional 12 mol CeO2-stabilised tetragonal zirconia polycrystalline (TZP). The toughness could be further increased up to 14.3 MPad m 1/2 by the addition of 2 wt.% Al2O3 to the starting powder.

A Dy-α-sialon with a composition of Dy 0.33 Si 9.3 Al 2.7 O 1.7 N 14.3 was prepared by hot pressing at 1800 °C with an intermediate dwelling at 1550 °C or 1650 °C during the sintering. The microstructures and optical properties of the sintered specimens were studied. The results showed that an intermediate dwelling stage in the hot pressing was beneficial to densification of the specimens and could suppress the development of elongated α-sialon grains, but it had a negative effect on hardness and fracture toughness. Besides, the intermediate dwelling process also improved the optical transmittance and the maximum infrared transmission, which is attributed to the increase in bulk density and the development of a homogeneous equi-axed microstructure.

Y0.67Si9Al3ON15 and Dy0.4Y0.3Si8.85Al3.15O1.05N14.95 ceramics were prepared by Spark Plasma Sintering (SPS) with and without heat treatment at 1700°C for 7 h or 17 h, and their optical transmittance were investigated over the wavenumber range 4000–1500 cm−1. The results showed that the assemblages of the SPS-ed samples consisted of single crystallized α-sialon phase in both compositions. EDS analysis indicated that α-sialon was mainly stabilized by Dy3+ in the multi-cation (Dy,Y)-α-sialon composition. The SPS-ed specimens showed relatively high optical transmission properties, and the maximum transmittance reached around 65% at 2800 cm−1 for 0.5 mm thick specimens of both compositions. The 7 h heat treatment caused the formation of small amount of melilite phase, resulting in non-uniform microstructure and decrease in optical transmittance. Extended heat treatment for 17 h led to more homogenous microstructure and increased the transmittance to some extent. Less melilite was formed in the multi-cation (Dy,Y)-α-sialon composition than in the single cation Y-α-sialon after heat treatment, and the transmittance of (Dy,Y)-α-sialon was also higher than that of Y-α-sialon.

By hot pressing at 1800 °C for 2 h and heat treatment at 1700 °C for 20 h, infrared transmittance of Y- and Dy-α-sialon, (Y1017 and Dy1017, corresponding to m =1, n =1.7; Y2010 and Dy2010, corresponding to m =2.0, n =1.0), according to formula M m /3 Si 12−( m + n ) Al m + n O n N 16- n , in 1500–4000 cm −1 (6.6–2.5 μm) were studied. The results showed that the infrared transmittance of around 45% and 50% could be obtained for hot-pressed Y2010/Dy2010 and Y1017/Dy1017 samples respectively in 0.5 mm thickness in the range of 2000–3300 cm −1 . The higher optical transmittance values of Y1017/Dy1017 than that of Y2010/Dy2010 were mainly resulted from the fact that the phase assemblage of Y1017/Dy1017 consisted of only crystallized α-sialon phase and small amount of melilite phase was formed for Y2010/Dy2010. The heat treatment encouraged the grain growth of α-sialon. Optical transmittances of Y1017/Dy1017 slightly decreased after heat treatment however, especially for Dy1017, which is mainly due to the formation of a small amount of garnet phase. Whereas, the optical transmittance of Y2010/Dy2010 increased from 45% to 55% as a result of α-sialon growing up and intergranular glassy phase moving. The results indicated that a suitable composition to avoid the formation of the second crystalline phase after heat treatment is important to make α-sialon materials with good optical properties.

Bismuth titanate (Bi 4 Ti 3 O 12 ) platelets were obtained by the molten salt synthesis method in NaCl–KCl and Na 2 SO 4 –K 2 SO 4 fluxes, using an amorphous Bi 4 Ti 3 O 12 precursor and a mechanically mixed Bi 2 O 3 –TiO 2 mixture as the starting materials. It was found that the synthesizing temperature, salt species and starting materials could significantly affect the crystallization habit and morphology of Bi 4 Ti 3 O 12 platelets. Under the same processing conditions (i.e. in Na 2 SO 4 –K 2 SO 4 flux at 1000 °C), the Bi 4 Ti 3 O 12 platelets synthesized from the Bi 2 O 3 –TiO 2 mixture generally showed a smaller particle size than those synthesized using the amorphous Bi 4 Ti 3 O 12 precursor. The Bi 4 Ti 3 O 12 platelets prepared in the chloride flux were faceted along either (0 0 1) or (1 1 2) planes, while those prepared in the sulfate flux were solely (0 0 1) faceted. The former system also had smaller particle size than the latter. Furthermore, the salt content and the addition of plate-like Bi 4 Ti 3 O 12 seeds in the amorphous Bi 4 Ti 3 O 12 precursor showed a strong influence on the particle size of the synthesized Bi 4 Ti 3 O 12 platelets. The particle size of Bi 4 Ti 3 O 12 platelets prepared in the sulfate flux decreased as the mole ratio of the sulfate salts to Bi 4 Ti 3 O 12 was increased from 7.9 to 23.7. And the addition of 10 wt.% plate-like Bi 4 Ti 3 O 12 seeds into the amorphous Bi 4 Ti 3 O 12 precursor led to a significant increase in the particle size of the resulting Bi 4 Ti 3 O 12 platelets.

Bi4Ti3O12 powder was prepared by a co-precipitation method and characterized with DTA, XRD, IR, BET and TEM. The results indicated that the as-prepared precursor consisted of well-dispersed particles of about 10 nm. Bi4Ti3O12 phase began to form at 470 jC, and pure Bi4Ti3O12was obtained after being calcined at 550 jC for 0.5 h. The powder obtained at 600 jC showed a tetragonal symmetry and a plate-like morphology with particle size of about 100 nm. Bi4Ti3O12 ceramics with 97.2% of theoretical density can be obtained by pressure-less sintering at 750 jC for 1 h, indicating the excellent sinterability of the prepared powder. D 2002 Elsevier Science B.V. All rights reserved.

The formation behavior of SrxBa1−xNb2O6 (SBN) (x=0.4 and 0.6) was investigated by the normal solid-state reaction route using SrCO3, BaCO3, and Nb2O5 as the starting materials. The phase assemblage of the reaction products at different temperatures was identified by X-ray Guinier-Hagg camera film data and the unit cell dimensions of SrxBa1−xNb2O6 formed during reaction sequence were also precisely calculated based on film data. The results indicated that four intermediate phases, i.e. Ba5Nb4O15, Sr5Nb4O15, SrNb2O6 and BaNb2O6 were developed during the formation of SrxBa1−xNb2O6. SrxBa1−xNb2O6 formed by the reaction of SrNb2O6 and BaNb2O6, appeared at 900 °C, whose intensity subsequently increased with the increase of calcination temperature. SrxBa1−xNb2O6 was the only crystalline phase when the starting powder was calcined at 1100 °C for Sr0.4Ba0.6Nb2O6 composition and 1200 °C for Sr0.6Ba0.4Nb2O6 composition. It was also found that the lattice parameters of SrxBa1−xNb2O6 formed at different temperatures decreased with the increasing calcination temperature, implying that the solubility of SrxBa1−xNb2O6 changed and more Sr would be incorporated into SrxBa1−xNb2O6 with the increase of calcination temperature. Based on the above results, the corresponding formation mechanism of SrxBa1−xNb2O6 phase was modified.

Bismuth oxide (Bi2O3) needles were fabricated hydrothermally by altering the starting materials, mineralizers, reaction temperature and time. The non-agglomerated Bi2O3 needles with a monoclinic structure were prepared using Bi(OH)3 in an alkali solution. The hydrothermal reaction took place at 120 °C in 0.5 h. The length and aspect ratio (length over diameter) of the needles are in the ranges of 6–70 and 5–16 μm, respectively. With the rising reaction temperature and time, the diameter of Bi2O3 needles increased, while the aspect ratio decreased. The formation mechanism of Bi2O3 needles is discussed in the paper.

A proper powder preparation was used to develop α-Sialon single crystals as seeds. The microstructure and fracture toughness of seed-containing α-Sialon ceramics sintered by hot-pressing were investigated. The specimen without seeding consisted of fine grains and a small amount of coarse grains. Specimens seeded with α-Sialon single crystal particles presented a large amount of elongated α-Sialon grains. The aspect ratio and the amount of elongated α-Sialon grains can be tailored by using different sizes and amounts of the seeds. The fracture toughness of seed-containing α-Sialon ceramics is improved, which is attributed to grain pullout and bridging of elongated grains.

Three kinds of additives were added into Ca α-sialon, (Ca 0.6 Si 10.2 Al 1.8 O 0.6 N 15.4 ). It was found that the additives resulted in more of the liquid phase during sintering, which promoted the completion of the α→α′ transformation and the development of the elongated morphology characteristic of α-sialon grains. Comparison was made concerning the effect of the amount and type of additives on the microstructural development of Ca α-sialon. Phase and property analysis was also carried out on the hot-pressed samples.

Ca-α-sialon compacts pressureless-sintered to intermediate temperatures, which consisted of both α-sialon and unreacted α–Si3N4 grains, were investigated with transmission electron microscopy for an overall composition Ca1.8Si6.6Al5.4O1.8N14.2. Special attention was paid to identification of the possible crystallographic orientation between a-sialon and the α–Si3N4 particles. In contrast to the frequently occurring heteroepitaxial nucleation of α-sialon in rare-earth-doped samples with low x values, this study showed that most of the newly formed α-sialon grains had no epitaxial orientation relationship with the α–Si3N4 particles, suggesting nonepitaxial heterogeneous nucleation to be a more probable mechanism for the Ca–α-sialon phase with high Ca concentrations.

Two R-α-sialon (R0.6Si9.3Al2.7O0.9N15.1, R=Nd and Er) compositions were first fired at 1750°C/25 min and 1650°C/2 h respectively for completion of the α → α′ phase transformation. Elongated α-sialon grain morphology was developed in both samples after being re-fired at 1800°C for different periods of time. The growth in width of R-α-sialon grains is controlled by diffusion in the liquid, while the length growth tends to be interfacial reaction controlled. The anisotropic growth of R-α-sialon is attributed to the large difference in the growth rate constant between the length and the width directions of the grain.

The influence of processing on microstructural development of Ca α-sialon composition (CaxSi12−3xAl3xOxN16−x) with x = 1.8 was studied by dwelling at different intermediate temperatures before reaching the final sintering temperature. The microstructural observation results have showed the different aspect ratios of elongated grains obtained by the various processing conditions, reflecting the effect of the number of nuclei of α-sialon on morphology of grains during sintering. Improved toughness was achieved by applying low temperature dwelling for Ca α-sialon compositions with low x values. The toughness showed an increase of 33% and 16% for x = 0.6 and 1.0 compositions respectively with middle dwelling processing at 1350°C for 3 h before reaching 1750°C for 1 h by hot-pressing.

Multi-cation α-Sialons ceramics containing calcium and magnesium for the compositions ((Ca,Mg)xSi12 − 3xAl3xOxN16 − x), where x equals 0.6 and 1.0 with the different atomic ratios of Ca to Mg, were fabricated by hot-pressing. An exploration for Ca,Mg-α-Sialon involving phase compositions, cell dimensions, microstructure and mechanical properties were carried out in the present work.

ZrB 2 –SiC composite powders have been synthesized by combustion synthesis in air, using a mixture of Zr, B 4 C and Si as raw materials. It was found that the air atmosphere has played an important role in the ignition process of the SHS reaction. Three other kinds of ZrB 2 –SiC–ZrC composite powders with different ZrC content were also synthesized, and the ignition time has been measured for better understanding the ignition mechanism. As a result, the composite powders with particle size smaller than 1 µm and oxygen content as low as 0.4 wt.% were obtained.

We have previously reported the preparation of ZrB 2 –SiC composite from the pyrolyzed mixtures of polycarbosilane-coated ZrB 2 powders via hot pressing at 2073 K for 60 min under a pressure of 20 MPa in an argon atmosphere. In the present paper, Si additive was introduced into the ZrB 2 –SiC composite fabricated with the same processing conditions, in an attempt to convert carbon impurities introduced by the pyrolysis of polycarbosilane into SiC. The phase composition, microstructure and mechanical properties were characterized for the composite. Analysis showed that the Si additive could react with carbon to produce the SiC. In comparison with our previously published results, the addition of Si powder enhanced the micro-hardness of the composite, and did not have significant effect on fracture toughness.

ZrB2-SiC based composites with 0,5 and 15 vol% addition of ZrC were synthesized via reactive hot pressing at 1800°C using Zr, Si and B4C as raw materials. The mechanical properties of the composites were investigated. The composite of ZSC15 that contained 15 vol% of ZrC has the highest hardness. ZSC5 with 5 vol% of ZrC owns a most homogenous microstructure and the highest fracture toughness and flexural strength.

Rietveld refinements have been used to determine the structure of Y2Si3O3N4 from X-ray data and Nd2Si2.5Al0.5O3.5N3.5 from neutron powder diffraction data. The refinements show that in the melilite phase Y2Si3O3N4 and melilite solid solution Nd2Si2.5Al0.5O3.5N3.5 the distributions of cations and anions are almost identical. They are analogous to the akermanite (Ca2MgSi2O7) structure, with Si/Si,Al atoms at the origin and centre of the unit cell and with four N/N,O atoms forming the SiN4/(Si,Al)(N3.5O0.5) tetrahedra which share corners with SiO2N2/(Si,Al)O2.25N1.75 tetrahedra to form a continuous sheet structure. Each Y3+ or Nd3+ ion is surrounded by eight N/O atoms forming the coordination polyhedron in Y2Si3O3N4 and Nd2Si2.5Al0.5O3.5N3.5 respectively. The arrangement of Al, Si atoms in the tetrahedra in Nd2Si2.5Al0.5O3.5N3.5 structure is also discussed.

We report the glass-forming region at 1700°C for N-rich compositions of the Sm-Si-Al-O-N system

Rietveld refinements have been used to determine the cation distributions in three (Mg,Co) orthovanadates (X-ray data) and in Co 1.5 Ni 1.5 (VO 4 ) 2 (neutron powder diffraction data). The compounds crystallize in the orthorhombic space group Cmca(Z=4), with two distinct octahedral metal sites having point symmetries 2/m and 2. The refinements show that in all solid solutions the cation distributions are almost random, with a slight preference of Co 2+ for the most symmetric site.