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The authors acknowledge the ISIS Neutron and Muon User Source for access to beamtime on HRPD (RB1520166 and RB1720182) [39,40]. We are also thankful for access to the instrumentation in the ISIS Materials Characterisation laboratory. We are grateful to Professor Mike Reece at Queen Mary University London for access to the spark plasma sintering (SPS) instrumentation and for assistance preparing pellets. LJV and JAM are grateful for the award of EPSRC DTA studentships. Ca2Mn3O8 forms a delafossite-related layered structure, which crystallises with monoclinic C2/m symmetry. Compared with the delafossite-structure, the MnO6 layers in Ca2Mn3O8 exhibit an ordered cation void which forms a magnetic ‘bow-tie’ like connectivity of Mn4+ ion layers separated by Ca2+ ions. In-situ variable temperature diffraction data demonstrates that the structure is robust up to a temperature of approximately 1173 K before the material decomposes into the perovskite, CaMnO3 and marokite, CaMn2O4 phases. Simultaneous thermal analysis suggests that a very small amount of water remains within the layers post synthesis. Impedance spectroscopy indicates that Ca2Mn3O8 is an electronic conductor in the range ∼400–700 K with an activation energy of 0.50 ± 0.01 eV. Postprint

We thank the Science and Technology Facilities Council (STFC) for the provision of neutron diffraction facilities at ISIS (HRPD experiment RB1710021, https://doi.org/10.5286/ISIS.E.RB1710021) and the School of Chemistry, University of St Andrews for funding of a studentship to JAM through the EPSRC doctoral training grant (grant No. EP/K503162/1). The hexagonal tungsten bronze RbNbW2O9 is shown, by variable-temperature powder neutron diffraction and symmetry-mode analysis, to display a significantly different phase transition sequence compared to the related CsNbW2O9 composition. At ambient temperature, RbNbW2O9 adopts the polar orthorhombic space group Cmc21. Upon heating, the thermal evolution of the crystal structure proceeds via two transitions. These correspond to sequential loss of two distinct octahedral tilting modes, leading to space group P63mc at around 655K, and space group P6mm near 700 K. The polar distortion is retained up to the highest temperature studied here. The differences in structural behaviour between the proper ferroelectric RbNbW2O9 and the improper ferroelectric CsNbW2O9 emphasises the need for careful crystallographic analyses of materials of this type. Postprint

We have synthesized ceramic specimens of the tetragonal tungsten bronze ${\mathrm{K}}_{3}\mathrm{L}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{a}}_{5}{\mathrm{O}}_{15}$ (KLT) and characterized its phase transition via x-ray diffraction, dielectric permittivity, resonant ultrasonic spectroscopy, and heat capacity measurements. The space group of KLT is reported as both $P4/mbm$ and $Cmmm$ with the orthorhombic distortion occurring when there are higher partial pressures of volatile K and Li used inside the closed crucibles for the solid state synthesis. The data show strong relaxor behavior, with the temperature at which the two dielectric relative permittivity peaks decreasing, with $104\ensuremath{\ge}{T}_{m1}\ensuremath{\ge}69\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and $69\ensuremath{\ge}{T}_{m2}\ensuremath{\ge}46\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ as probe frequency $f$ is reduced from 1 MHz to 316 Hz. $F$ tests show that the data satisfies a Vogel-Fulcher model better than Arrhenius with an extrapolated freezing temperature for ${\ensuremath{\varepsilon}}^{\ensuremath{'}}$ and ${\ensuremath{\varepsilon}}^{\ensuremath{''}}$ of ${T}_{f1}=+15.8$ and $\ensuremath{-}11.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{f2}=\ensuremath{-}5.0$ and $\ensuremath{-}15.0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\phantom{\rule{4pt}{0ex}}f\ensuremath{\rightarrow}0$ (tending to dc). This difference between ${T}_{f}$ from real and imaginary values, albeit counterintuitive, is mandatory, according to the theory of Tagantsev. Therefore, by tuning frequency, the transition could be shifted to absolute zero, suggesting KLT has a relaxor-type quantum critical point. In addition, we have reanalyzed the conflicting literature for $\mathrm{P}{\mathrm{b}}_{2}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{7}$ pyrochlore which suggests that this also has a relaxor-type quantum critical point since the freezing temperature from the Vogel-Fulcher fitting is below absolute zero. Since the transition temperature evidenced in the dielectric data at approximately 100 kHz shifts below 0 K for very low frequencies, this transition would not be seen with heat capacity data collected in the zero-frequency (dc) limit. Both of these materials show promise for possible new relaxor-type quantum critical points with nonperovskite based structures.

We thank the Science and Technology Facilities Council for access to the ISIS neutron facility (RB1210111). CMK would like to thank the EPSRC for provision of a studentship via the doctoral training grant (EP/K503162/1). A detailed powder neutron diffraction study of Bi0.7Nd0.3FeO3 shows it is a Gz-type antiferromagnet adopting the space group Pn′ma ́. Detailed structural analysis using both Rietveld and symmetry mode analysis indicates that competition between geometric effects and magnetic superexchange results in unusual evolution of octahedral tilting and structural distortion on cooling. These changes are manifested in both the electrical response, which displays relaxer-type behaviour, and also in a region of negative thermal expansion below 200 K. Postprint

The authors demonstrate room-temperature multiferroic (ferroelectric-ferromagnetic) behavior in lead titanate, in which about 30% of the titanium is substituted by palladium. Palladium is an unusual element with an instability to ferromagnetism under some special conditions. Typically, it has not been easy to incorporate it into perovskite oxides -- the most common structure for ferroelectric device materials. Although palladium is expensive, commercial devices like ferroelectric memories often use very thin films, for which the added cost of palladium is negligible. Thus, the present study combines basic new physics of the first multiferroic palladium compounds with potential commercial devices.

Hexagonal ferrites do not only have enormous commercial impact ({\pounds}2 billion/year in sales) due to applications that include ultra-high density memories, credit card stripes, magnetic bar codes, small motors and low-loss microwave devices, they also have fascinating magnetic and ferroelectric quantum properties at low temperatures. Here we report the results of tuning the magnetic ordering temperature in PbFe$_{12-x}$Ga$_x$O$_{19}$ to zero by chemical substitution $x$. The phase transition boundary is found to vary as $T_N \sim (1-x/x_c)^{2/3}$ with $x_c$ very close to the calculated spin percolation threshold which we determine by Monte Carlo simulations, indicating that the zero-temperature phase transition is geometrically driven. We find that this produces a form of compositionally-tuned, insulating, ferrimagnetic quantum criticality. Close to the zero temperature phase transition we observe the emergence of an electric-dipole glass induced by magneto-electric coupling. The strong frequency behaviour of the glass freezing temperature $T_m$ has a Vogel-Fulcher dependence with $T_m$ finite, or suppressed below zero in the zero frequency limit, depending on composition $x$. These quantum-mechanical properties, along with the multiplicity of low-lying modes near to the zero-temperature phase transition, are likely to greatly extend applications of hexaferrites into the realm of quantum and cryogenic technologies., Comment: 17 pages. Final version as published

We report a paradigm of ferrielectricity in a single-phase crystal, tris-sarcosine calcium chloride [TSCC; ${(\mathrm{C}{\mathrm{H}}_{3}\mathrm{NHC}{\mathrm{H}}_{2}\mathrm{COOH})}_{3}\mathrm{CaC}{\mathrm{l}}_{2}]$. Ferrielectricity is well known in smectic liquid crystals but almost unknown in true crystalline solids. C. F. Pulvari [Ferrielectricity, Phys. Rev. 120, 1670 (1960)] reported it in 1960 in mixtures of ferroelectrics and antiferroelectrics (AFEs), but only at high fields. Tris-sarcosine calcium chloride exhibits a second-order displacive phase transition near ${T}_{\mathrm{c}}=130\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ that can be lowered to a quantum critical point at 0 K via Br or I substitution and phases previously predicted to be AFE at high pressure and low temperatures. Unusually, the size of the primitive unit cell does not increase in the so-called AFE phase. We measure hysteresis loops and polarization below $T=64\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and provide clear Raman evidence for this paraelectric-ferrielectric transition.

Guest Editors Caroline Kirk, Finlay Morrison, Jan Skakle, Derek Sinclair and John Irvine introduce this Journal of Materials Chemistry A themed issue on advances in solid state chemistry and its applications, in celebration of Professor Tony West’s 70th birthday.

JAM would like to acknowledge the School of Chemistry, University of St Andrews for the allocation of a PhD studentship through the EPSRC doctoral training grant (EP/ K503162/1). The work carried out at the University of St Andrews and Queens University Belfast was carried out as part of an EPSRC-funded collaboration (EP/P02453X/1 and EP/P024637/1). The work carried out at the University of Glasgow was carried out as part of the EPSRC-funded CDT in Photonic Integration and Advanced Data Storage (EP/L015323/1). TTT and PSH thank the Welch Foundation (Grant E-1457) and NSF (DMR-1503573) for support. Since the observation that the properties of ferroic domain walls (DWs) can differ significantly from the bulk materials in which they are formed, it has been realized that domain wall engineering offers exciting new opportunities for nanoelectronics and nanodevice architectures. Here, a novel improper ferroelectric, CsNbW2O9, with the hexagonal tungsten bronze structure, is reported. Powder neutron diffraction and symmetry mode analysis indicate that the improper transition (TC = 1100 K) involves unit cell tripling, reminiscent of the hexagonal rare earth manganites. However, in contrast to the manganites the symmetry breaking in CsNbW2O9 is electronically driven (i.e., purely displacive) via the second-order Jahn-Teller effect in contrast to the geometrically-driven tilt mechanism of the manganites. Nevertheless CsNbW2O9 displays the same kinds of domain microstructure as those found in the manganites, such as the characteristic six-domain "cloverleaf" vertices and DW sections with polar discontinuities. The discovery of a completely new material system, with domain patterns already known to generate interesting functionality in the manganites, is important for the emerging field of DW nanoelectronics. Postprint

The energetics of La-doping in BaTiO3 are reported for both (electronic) donor-doping with the creation of Ti3+ cations and ionic doping with the creation of Ti vacancies. The experiments (for samples prepared in air) and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La-doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen-loss, which leads to the creation of Ti3+ cations. Simulations show that oxygen-loss is much more favorable in the ionic-doped system than undoped BaTiO3 due to the unique local structure created around the defect site. These findings resolve the so-called “donor-doping” anomaly in BaTiO3 and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO3 thermistors. Postprint

This work was supported by the Leverhulme Trust (RPG-2012-576). Single crystals of SnSe have been reported to have very high thermoelectric efficiencies with a maximum figure merit zT = 2.5. This outstanding performance is due to ultralow thermal conductivities. We report on the synthesis of highly textured polycrystalline SnSe ingots with large single-crystal magnitude power factors, S2/ρ = 0.2-0.4 mW m-1 K-2 between 300-600 K, increasing to 0.9 mW m-1 K-2 at 800 K, and bulk thermal conductivity values κ300K = 1.5 W m-1 K-1. However, small SnSe ingots, which were measured in their entirety, were found to have a substantially reduced κ300K = 0.6 W m-1 K-1. Microscopy and diffraction revealed two distinct types of texturing within the hot-pressed ingots. In the interior, large coherent domains of SnSe platelets with a ∼45° orientation with respect to the pressing direction are found, while the platelets are preferentially oriented at 90° to the pressing direction at the top and bottom of the ingots. Fitting the κ(T) data suggests an increase in defect scattering for the smaller ingots, which is in keeping with the presence of regions of structural disorder due to the change in texturing. Combining the measured S2/ρ with the bulk ingot κ values yields zT = 1.1 at 873 K. Postprint

We thank EPSRC for funding, STFC for providing neutron facilities and Diamond Light Source for provision of synchrotron facilities. We thank Dr Chiu Tang for assistance at Diamond and Dr A. Kusmartseva (University of Edinburgh) for assistance with the SQUID measurements. FDM thanks the Royal Society for a Research Fellowship. The hexagonal ferrites MFeO3 (M=Y, Yb, In) have been studied using a combination of neutron and X-ray powder diffraction, magnetic susceptibility, dielectric measurements and 57Fe Mössbauer spectroscopy. This study confirms the previously reported crystal structure of InFeO3 (YAlO3 structure type, space group P63/mmc), but YFeO3 and YbFeO3 both show a lowering of symmetry to at most P63cm (ferrielectric YMnO3 structure type). However, Mössbauer spectroscopy shows at least two distinct Fe sites for both YFeO3 and YbFeO3 and we suggest that the best model to rationalise this involves phase separation into more than one similar hexagonal YMnO3-like phase. Rietveld analysis of the neutron diffraction data was carried out using two hexagonal phases as a simplest case scenario. In both YFeO3 and YbFeO3, distinct dielectric anomalies are observed near 130 K and 150 K, respectively. These are tentatively correlated with weak anomalies in magnetic susceptibility and lattice parameters, for YFeO3 and YbFeO3, respectively, which may suggest a weak magnetoelectric effect. Comparison of neutron and X-ray powder diffraction shows evidence of long-range magnetic order in both YFeO3 and YbFeO3 at low temperatures. Due to poor sample crystallinity, the compositional and structural effects underlying the phase separation and possible magnetoelectric phenomena cannot be ascertained. Postprint

The tetragonal tungsten bronze (TTB) structure multiferroic fluoride K0.58FeF3 (alternatively K3–xFe5F15 (x ≈ 0.1)) has been studied using variable temperature high-resolution synchrotron powder X-ray diffraction. The as-made sample occurs as a mixture of two closely related TTB phases, one tetragonal (aTTB ≈ 12.68 A, cTTB ≈ 3.98 A) and one orthorhombic (a ≈ 12.70 A, b ≈ 12.66 A, c ≈ 3.98 A). Heating above 340 K produces a single metrically tetragonal phase, but the phase separation reproducibly reappears on cooling. Below 300 K the tetragonal phase undergoes a structural phase transition to a second orthorhombic phase (with a ≈ b ≈ √2 aTTB) and these two different orthorhombic phases continue to coexist down to at least 125 K. These results are discussed in the context of previous studies, which sometimes suggest conflicting structural details within this system.

The crystal structure of the S = 1/2 kagome system Cs2ZrCu3F12 has been determined at both 295 and 125 K via single-crystal X-ray diffraction. A first-order structural phase transition is seen near 225 K, confirmed by variable-temperature synchrotron powder diffraction studies, in which the structure transforms from rhombohedral (R3m) to monoclinic (P21/m). A corresponding abrupt change in dielectric constant is observed near the same temperature. The phase transition is driven by a dramatic change in coordination around the Zr site, which changes from regular octahedral in the rhombohedral phase to a seven-coordinate environment in the monoclinic phase. This leads to a severe buckling of the copper fluoride kagome layers and significant changes in the geometry around the Cu sites, which correlates with both the observed dielectric anomaly and a previously observed anomaly in magnetic susceptibility. It is suggested that this structural phase transition ultimately permits long-range antiferromagnetic ord...

The cryolite phase (NH4)3FeF6 has been studied using variable temperature X-ray powder diffraction, together with dielectric and magnetic measurements, in order to resolve its potential multiferroic properties. The X-ray data reveal a direct transition from cubic to triclinic symmetry between 270 and 250 K, consistent with earlier reports of a ferroelectric TC of 264 K. The crystal structure of the low temperature phase has been modelled approximately in a pseudo-tetragonal, triclinic space group F1, a = 8.98258(9) A, b = 8.99631(9) A, c = 9.27961(9) A, α = 90.039(1)°, β = 90.451(1)° and γ = 90.339(1)° (at 100 K) but weak superlattice reflections remain unindexed, suggesting the true unit cell to be larger. Electrical measurements indicted an increase in capacitance above 260 K, however this was accompanied by an increase in dielectric loss and so is inconclusive evidence of a ferroelectric–paraelectric phase transition. Polarisation-field loops were not possible due to overriding leakage. Magnetic susceptibility data show paramagnetic behaviour with no indication of long-range order above 2 K, and only weak antiferromagnetic interactions (θ ∼ −7 K). This therefore clearly demonstrates the material is not multiferroic.

The layered perovskite CsBiNb2O7 has been reinvestigated in the light of recent predictions of ferroelectric behavior. Variable temperature powder neutron diffraction experiments show that this material retains polar symmetry (space group P2(1)am) up to at least 900 degrees C, with only a slight decrease in net polarization up to this temperature, calculated using a simple ionic model. However, ac impedance measurements show only a modest dielectric constant (19-27) throughout the temperature range studied. Moreover, convincing evidence for ferroelectric behavior in polarization-electric field loops could not be obtained because of overriding electrical conductivity. Further measurements of electrical behavior, at different temperatures and under both "wet" and "dry" atmospheres, provide clear evidence that CsBiNb2O7 is hygroscopic, with water uptake resulting in significant protonic conductivity. Even in the "dehydrated" state the material is a semiconductor with a modest conductivity of 3.8 mu S cm(-1) at 250 degrees C. Although ferroelectric behavior is not observed in the samples studied here, we cannot rule out the possibility that ferroelectricity might be obtained in samples processed under appropriate conditions.