We report the crystal structure and electric transport and/or dielectric properties of the Ba(PO4)2M2O3 compounds, for M = Ta, Nb as well as the mixtures Ta/Nb, W/ Nb and W/Ta. While the existence of these phases has already been reported in parts, their accurate crystal structures remained unclear but suggest the R-3m space group for M = Ta and Nb. Here the crystal symmetry was revised to the chiral R32 space group, as confirmed by SHG. Furthermore, looking at them from the new viewpoint of the only recently discovered tungsten counterpart (certified R-3m symmetry) opens wide perspectives on materials properties of this family of compounds. For the d0 Nb/Ta5+ phases, we confirm that they are large-gap semiconductors (between 3.7 and 4.8 eV) with weak frequency dependence of the permittivity and low dielectric loss. For mixed Nb/Ta semiconductors the compositional dependence of the bandgap (Eg) and the dielectric constant (E'r) have been analyzed using a bowing parameter and a Clausius-Mosotti law, respectively. For M = Ta, the inversely related Eg and E'r values stand out of the global behavior of the solid solution. The analogy with the M = W case is all the more interesting, since Ba(PO4)2W2O3 is a rare example of a W compound in a single 5+ valence. It constitutes the m = 2 member of the recently-discovered [Ba(PO4)2][WmO3-m] layered-monophosphate bronze series (L-MPTB's). In this subclass, the occurrence of genuine 2D robust metallicity confined in thin areas of their crystal structures validate exotic electronic features hold by the (d0/d1) mixed W5/6+ valence imposed by the stoichiometry. Our work suggests the miscibility of electronically inactive d0 Ta/Nb5+ and d1 W5+ species in an extended compositional phase diagram. The possibility to tune and further confine the electron density of chemically modified L-MPTB is discussed towards intriguing opportunities.

Alkali-metal doping of layered oxychalcogenides Sr 3 Sc 2 O 5 Cu 2 Ch 2 significantly increases conductivity, with Na-doped Sr 3 Sc 2 O 5 Cu 2 Se 2 exhibiting exceptionally high p-type conductivity, marking it as a promising transparent conductor.

We report a comprehensive study of the structural and magnetic properties of CuCoFe2O5, a CaFe3O5-type mixed-metal oxide obtained at 20 GPa and 1000 degrees C. Room-temperature single-crystal and powder diffraction measurements confirm the orthorhombic Cmcm framework, with Cu2+ occupying a distorted CuO4+2 trigonal prismatic site and Co2+/Fe3+ residing in edge- and corner-sharing octahedral environments. Bond valence and anisotropic displacement analyses reveal pronounced local distortions around Cu2+, consistent with Jahn-Teller activity. Magnetic measurements indicate two successive transitions: a high-temperature antiferromagnetic order at T N1 = 195 K and a low-temperature spin reorientation at T N2 = 77 K of structural origin, the latter accompanied by a strong bifurcation between field- and zero-field-cooled magnetization curves. Neutron diffraction and DFT+U calculations demonstrate that the magnetic structure comprises competing Fe-Fe, Fe-Co, and Co-Cu interactions, leading to a canted ferrimagnetic state at low temperature. Moderate magnetic frustration (f approximate to 3.6) and strong spin-orbit coupling of Co2+ stabilize large coercivity (similar to 6.7 T at 2 K). These results highlight the intricate interplay between structural distortions, cation disorder, and competing exchange pathways in determining the complex magnetic ground state of CuCoFe2O5.

[Bi2O2][MnF4] is an Aurivillius oxyfluoride phase with Mn2+ cations filling the one-layer perovskite subunits. Its synthesis as a single-phase material by a high-temperature solid-state route is complicated by the high volatility of fluorine in the used precursors and by its limited thermal stability above 400 degrees C. Its crystal structure was determined using synchrotron and neutron diffraction data. It shows structural singularities, highlighted by the anion positional disorder inherent to the I4/mmm space group, despite the evidence of strong local axial and equatorial octahedral tilts along with clues for ideal F- and O2- total segregation in the two-layered subunits. Contrarily to the M = Ni2+, Co2+, and Fe2+/3+ analogs, where similar tilts order in various supercells, for M = Mn2+ only short-range ordering (SRO) was detected by electron diffraction. This was rationalized in terms of the d5 ion isotropic nature for which weak distortions of the crystal field do not have an energy penalty and favor local octahedral defects acting as antiphase boundaries between nanometric SRO domains. DFT calculations validate equiprobable tilted [MnF6] arrangements in the perovskite layers and suggest the possibility for an O <-> F exchange between the [Bi2O2] and [MnF4] modules, supported by HAADF imaging. The presence of oxygen and vacancies in the perovskite slabs was also detected in samples prepared at higher temperatures. [Bi2O2][MnF4] magnetic structure shows a colinear antiferromagnetic arrangement below T N = 19.8 K, with Mn2+ spins (S = 5/2, L = 0) aligned parallel to the c-axis (M z = 3.8(2) mu B/Mn). This is again in contrast with the systematic spin-canting responsible for weak ferromagnetism in the previous M2+ Aurivillius compounds. Finally, the modest fluoride ionic mobility was measured in a small temperature range, restricted by the stability of the compound in air.

In this work, nine new synthetic alkali-and rare-earth metal crichtonite-type compounds withAB3C18O38 general formula are presented. The effect on the magnetic properties as consequence of the substitution of divalent and trivalent cations in the A site, as well as the introduction of iron at the B and C sites is studied. Ferrimagnetic behavior was identified in the crichtonites CaMn3Ti18O38, BaMn3Ti18O38 and CaFe3Ti18O38, which changes to a spin-glass type for the BaMn3Ti14Fe4O38 and A-trivalent REMn3Ti18O38 (RE = La, Ce and Nd), LaFe3Ti18O38 and LaMn3Ti13Fe5O38. A comprehensive comparison of the AC and DC magnetic measurements for whole series along with the FiM structure obtained by neutron powder diffraction is discussed. These results will expand the comprehension on this almost unexplored magnetic family of compounds.

The mixed valent high‐pressure oxide K₀.₇ 5 Li 2 Cr₆ 3.54 +O 12 is synthesized at 12 GPa and 1373 K. Synchrotron X‐ray and powder neutron diffraction (PND) reveal a P6 3/m average structure. Analogous to hollandites, CrO 6 octahedra form a corner‐sharing double‐chain framework, this creates two types of channels respectively occupied by Li + and K + . However, electron microscopy and pair distribution function analysis suggest the loss of correlation in and between the K + partially occupied channels. A magnetostrictive paramagnetic‐to‐antiferromagnetic transition occurs at T N = 75 K, where low temperature PND reveals a propagation vector k = [⅓ ⅓ ¼] realising a commensurate helical structure with 45° rotation between Cr 3 ⁺ and Cr⁴⁺ spins in zig‐zag ladders, stabilized by competing ferromagnetic and antiferromagnetic exchanges. Density functional theory calculations highlight the critical role of the c ‐axis compression in determining the magnetic direct exchange interactions thus revealing the ferromagentic‐correlated paramagnet → charge‐ordered antiferromagnetic transition, reminiscent to the pressure induced FM to AFM transition in the related K 2 Cr 8 O 16 hollandite. Despite hidden long‐range charge order due to K⁺ disorder, K₀.₇ 5 Li 2 Cr₆ 3.54 +O 12 exhibits strong spin‐lattice coupling, where a slight change in the structure has a huge impact in the properties.

We studied the structural and magnetic properties of the double perovskite oxides containing mercury, A2MnTeO6 (A = Hg, Hg0.8Ca0.2, and Ca). Hg2MnTeO6 and (Hg0.8Ca0.2)2MnTeO6 crystallize in a rhombohedral phase with R3 space group, while Ca2MnTeO6 crystallizes in a monoclinic phase with P21/n space group. The magnetic susceptibility, specific heat, and neutron diffraction measurements show that, upon cooling, the commensurate antiferromagnetic (AFM) order occurs at 10 K, 8 K, and 11 K for A = Hg, Hg0.8Ca0.2, and Ca systems, respectively. The first-principles calculations agree well with the experimental results, showing that the type-I AFM spin arrangement, i.e., magnetic moments aligned ferromagnetically within the planes and antiferromagnetically between adjacent planes, has the lowest total energy compared to other types of long-range order. For all three systems, the magnetic entropy associated with the AFM order amounts to less than 80% of the theoretical value for the high-spin state of Mn2+, indicating magnetic frustration across the series. Additionally, an incommensurate order at 5 K was identified by neutron diffraction for Hg2MnTeO6, which can be described by helicoidal spin order. This observation makes Hg2MnTeO6 one of the most complex magnetic systems among double perovskite oxides.

A series of mixed-metal Aurivillius oxyfluorides of the ideal formula [Bi2O2][Fe1-xMxF4] was synthesized by hydrothermal synthesis with M = Mn, Co, and Ni. We first re-examined the Fe-only compound and deduced that despite the observation of inhomogeneous lattice parameters between batches, the iron valence remains constant around Fe∼2.5+ in all samples measured using Mössbauer spectroscopy. The mixed valency charge compensation is mainly assigned to the formation of Bi vacancies. For the mixed Fe/M phases, the most common observation, using various diffraction techniques, of long-range ordering between tilted [(Fe,M)F6] octahedra in the perovskite layers is reminiscent of the Fe, Co, and Ni parent members. This validates the possibility of well-defined anion-ordering, despite the mixing of cations with different ionic radii. A qualitative matching between the lattice evolution along the Fe/M solid solutions and our DFT relaxed ideal models supports this idea. Differences in the magnetic structures are observed between the single-metal and the mixed Fe/M compositions, while retaining ordered magnetic structures and escaping spin-glass behavior despite disordered Fe/M ions. In contrast to the non-collinear antiferromagnetic spin arrangements obtained in most of the parent cases, the majority of the mixed Fe/M compounds show a collinear structure with spins aligned along the c-axis, similar to the single-metal M = Mn2+ (L = 0) case in which spin-orbit coupling is absent. This suggests the predominant role of the spin contribution to the ordering of the magnetic moments as soon as both Fe and M intervene.

The mixed valent high-pressure oxide K₀.₇5Li2Cr₆3.54+O12 is synthesized at 12 GPa and 1373 K. Synchrotron X-ray and powder neutron diffraction (PND) reveal a P63/m average structure. Analogous to hollandites, CrO6 octahedra form a corner-sharing double-chain framework, this creates two types of channels respectively occupied by Li+ and K+. However, electron microscopy and pair distribution function analysis suggest the loss of correlation in and between the K+ partially occupied channels. A magnetostrictive paramagnetic-to-antiferromagnetic transition occurs at TN = 75 K, where low temperature PND reveals a propagation vector k = [⅓ ⅓ ¼] realising a commensurate helical structure with 45° rotation between Cr3⁺ and Cr⁴⁺ spins in zig-zag ladders, stabilized by competing ferromagnetic and antiferromagnetic exchanges. Density functional theory calculations highlight the critical role of the c-axis compression in determining the magnetic direct exchange interactions thus revealing the ferromagentic-correlated paramagnet → charge-ordered antiferromagnetic transition, reminiscent to the pressure induced FM to AFM transition in the related K2Cr8O16 hollandite. Despite hidden long-range charge order due to K⁺ disorder, K₀.₇5Li2Cr₆3.54+O12 exhibits strong spin-lattice coupling, where a slight change in the structure has a huge impact in the properties.

The metamict fergusonite-(Y) with the formula (Y0.70Ln0.20Ca0.13U0.02Th0.02)∑1.07(Nb0.72Ta0.17W0.06Ti0.04)∑1(O3.97(OH)0.11F0.08Cl0.03) · 2.12H2O from the Blyumovskaya Pit, Ilmeny Mountains (Russia) was studied by the means of high-temperature X-ray diffraction, thermal analysis, Raman spectroscopy and microprobe analysis. Thermal expansion was studied for both tetragonal (α-fergusonite) and monoclinic (β-fergusonite) polymorphs. The expansion of β-fergusonite is anisotropic and strongly negative along the α33. In contrast, α-fergusonite exhibits a relatively isotropic thermal expansion upon heating. The volume CTE (αV) for β-fergusonite varies in the range 22.87(94)–75.4(2.5) × 10–6 ºC−1, whereas α-fergusonite has αV = 32.33(57)–31.66(49) × 10-6 ºC−1 in the temperature range 850–1200 °C. After heating to 1100 °C, the mineral develops a porous texture, and the radioactivity is reduced by 37

The newly discovered series of layered monophosphate tungsten bronzes (L-MPTB) [Ba(PO4)2]WmO3m-3 consist of m-layer-thick slabs of WO6 octahedra separated by barium-phosphate spacers. They display a 2D metallic behavior confined in the central part of the perovskite slabs. Here, we report the missing m = 2 member of this series, containing the rather uncommon W5+ oxidation state. We have analyzed its structure-property relationships in relation to the other members of the L-MPTB family. In particular, we have determined its crystal structure by means of single-crystal X-ray and electron diffraction and investigated its physical properties from resistivity, Seebeck-coefficient and heat-capacity measurements combined with first-principles calculations. All the L-MPTB compounds show metallic behavior down to 1.8 K without any clear charge-density-wave (CDW) order. The m = 2 member, however, displays an increased influence of the spacer that translates into anisotropic negative thermal expansion, reversed thermopower and reversed crystal-field splitting of the tungsten t2g orbitals. Our analysis of the full [Ba(PO4)2]WmO3m-3 series reveals a systematic and significant W off-centering in their octahedral coordination. We identify the resulting anti-polar character of these W displacements as the crucial aspect behind the 2D metallicity of these systems: It leads to the presence of bound charges whose screening determines the distribution of mobile charges, tending to accumulate at the center of the [WmO3-m] block. We argue that this mechanism is analogous to enhanced conductivity observed for charged domain walls in ferroelectrics, thus providing a general design rule to promote 2D metallicity in layered systems.

The synthesis, structural characterization and magnetic properties of the (Mn1/3R2/3)(2)(Mn1/3Sb2/3)(2)O-7 pyrochlore series are detailed herein. The R3+ (Rare earth) and Sb5+ cations occupy the A and B sites respectively, due to their charge/size ratio. Mn is found to be located in a 1/3 ratio in both positions with selective oxidation state due to size restrictions. Mixed valence in Mn2+/Mn3+ was confirmed from EELS spectroscopy. According to their geometric frustration and R3+-Mn2+/Sb5+-Mn3+ disorder, spin glass behavior is observed from dc and ac magnetic susceptibility data and no long-range magnetic order could be observed from NPD data down to 1.5 K. The magnetic entropy variation calculated from field dependent magnetization curves reveals large magnetocaloric effect (MCE) for these compounds, suggesting its relation to the dynamics of the frozen state. These values reach up to Delta SM similar to 20.7 J/kgK for the R=Gd sample, which is comparable to other materials with large MCE induced by FM order. This renders these materials competitive in magnetic refrigeration and opens the path for new enhanced magnetocaloric materials exploiting this mechanism.

We study the structural, magnetic and electronic properties of MFeNbO6 materials (where A = Ti, Zr or Hf). TiFeNbO6 crystallises with the disordered rutile structure with the P42/mnm space group. Increasing the ionic radii of the M4+ cation going from Ti4+ (0.605 Å) to Zr4+ (0.72 Å) and Hf4+ (0.710 Å) results in a buckling of the octahedral chains with these materials crystallising with the disordered α-PbO2 structure (space group Pbcn). In contrast, with previous reports for TiFeNbO6 we find no evidence of relaxor ferroelectric behaviour with all three materials instead exhibiting Maxwell-Wagner-like relaxation. Impedance spectroscopy confirms semiconductor behaviour. Magnetically, all three materials can be described by infinite irregular antiferromagnetic S = 5/2 chains with weak ferromagnetism below 10 K. These chains have random directions of propagation between nearest Fe-Fe neighbours consistent with the disordered nature of these materials. We observed a negative imaginary part of the ac susceptibility related to the defect magnetic topology.

Pressure evolution of the crystal structure and magnetism of the honeycomb alpha -RuBr 3 is studied using high-pressure x-ray diffraction, magnetometry, and density -functional band -structure calculations. Hydrostatic compression transforms antiferromagnetic alpha -RuBr 3 ( R 3 ) into paramagnetic alpha ' -RuBr 3 ( P 1 ) where short Ru-Ru bonds cause magnetism collapse above 1.3 GPa at 0 K and 2.5 GPa at 295 K. Below this critical pressure, the N & eacute;el temperature of alpha -RuBr 3 increases with the slope of 1.8 K / GPa. Pressure tunes alpha -RuBr 3 away from the Kitaev limit, whereas increased third -neighbor in -plane coupling and interlayer coupling lead to a further stabilization of the collinear zigzag state. Both alpha - and alpha ' -RuBr 3 are metastable at ambient pressure, but their transformation into the thermodynamically stable 0 polymorph is kinetically hindered at room temperature.

Over the past few decades Heusler alloys have provided a rich ground for exploration and discovery of half-metal candidates resulting in the progress of diverse spintronic applications. The current investigation in Heusler alloy Pd2MnIn demonstrates how the application of pressure could be used to introduce local deformational strain into the lattice and its effect of magneto-electronic properties in these systems. The results, showcasing high-pressure neutron diffraction experiments, X-ray diffraction and magnetisation studies, indicate that cycling to similar to 70 kbar irreversibly locally deforms the crystal lattice leading to the formation of ferromagnetic, glassy Kondo islands that persist up to high temperatures and ambient pressure conditions. The newly discovered strain-driven Kondo response could lead to breakthroughs in the search for half-metals, Kondo metals and other unconventional spintronic materials.

Aside from its economic value, davidite and its synthetic analogs may have potential applications in materials science. The unique properties of the crichtonite group minerals, including davidite-(La), make them attractive candidates for high-level waste (HLW) immobilization. We studied the thermal evolution of the metamict davidite-(La) from the Radium Hill, Australia. The investigation of the temperature-induced crystallization process was conducted, and the thermal expansion coefficients (TEC) for the recrystallized davidite (RD) were determined for the first time. Our results demonstrate that RD has relatively low TEC indicating its thermophysical stability. The following TECs of davidite- (La) for the temperature range 25–1200 °C were obtained: α a = α b = 9.96 (3) × 10–6 ºC−1; α c = 10.79 (4) × 10–6 ºC−1. The character of the thermal expansion is in agreement with the structure characterized by layers stacked along the c axis. The volume TEC αV = 24.81 (47)—36.80 (48) × 10–6 ºC−1. Davidite-(La) exhibits an almost isotropic thermal expansion and shows one of the most superior thermal performances in comparison to the other mineral-like phases utilized for the immobilization of HLW.

Over the past few decades Heusler alloys have provided a rich ground for exploration and discoveryof half-metal candidates resulting in the progress of diverse spintronic applications. The currentinvestigation in Heusler alloy Pd2MnIn demonstrates how the application of pressure could beused to introduce local deformational strain into the lattice and its effect of magneto-electronicproperties in these systems. The results, showcasing high-pressure neutron diffraction experiments,X-ray diffraction and magnetisation studies, indicate that cycling to ∼70 kbar irreversibly locallydeforms the crystal lattice leading to the formation of ferromagnetic, glassy Kondo islands thatpersist up to high temperatures and ambient pressure conditions. The newly discovered straindrivenKondo response could lead to breakthroughs in the search for half-metals, Kondo metals andother unconventional spintronic materials.

AbstractFe‐based mica minerals usually display two opposing magnetic ground states, either they behave as spin‐glasses or as layered ferrimagnets. No definite reason has been proposed as an explanation for this duality. This conundrum is unraveled by comparing the synthetic micas KFe3[MGe3]O10X2 with M═Fe and Ga, X═OH− and F−. Neutron diffraction demonstrates a 2D to 3D magnetic transition in KFe3[FeGe3]O10(OH)2 while just hints or no order at all are observed for the fluorides with M═Fe and Ga respectively. The 3D transition is triggered by the presence of iron in the intralayer tetrahedra. DFT+U calculations show that the magnetic exchange couplings between the previously believed solely magnetic octahedral layers would otherwise be frustrated without this intralayer iron.

In the 2D-Ising BaFe2(PO4)(2) ferromagnet (FM), the competition between giant magneto-crystalline anisotropy and strong FM couplings creates remarkably narrow magnetic domain walls. These domains freeze below T-F approximate to 15 K which is accompanied by progressive "soft-magnet" -> "super-hard magnet" transition at both sides of T-F. In the hard-regime, the coercive force is above 17 T at 2 K. At 5 K, we calculate a B.H-max figure of merit ( i.e. , energy product) of 6.5 MG Oe superior to what observed in most permanent magnet oxides. This allows for the printing above T-F and robust locking below T-F of any magnetization between the saturation limits. The outstanding Hc value surpasses the standards by far, and brings questions about origin of such unusually strong pinning effects. They have to be surely attributed to narrow domain walls, estimated to be only similar to 16 angstrom thick. Besides the observation of the domain wall structure by magnetic-force microscopy in various conditions, their thermodynamic signature appears in the specific heat data.

Two quaternary manganese selenites, A(2)(Mn2O)(SeO3)(3) (A = K, Rb), have been synthesized by hydrothermal reactions. They both crystallize in a complex triclinic (P-1) structure built of Jahn-Teller (JT) distorted Mn3+O4+2 octahedra, connected into nearly isosceles [Mn3O14] triangles, themselves arranged into so-called "sawtooth (ST) chains". The K and Rb compounds show subtle variations in the orientations of the MnO4 planes inside the elementary triangles. The ST chains are structurally and magnetically isolated by SeO3 groups and alkali cations. In the ST chains, predominant ferromagnetic interactions were calculated and verified experimentally, which finally order antiferromagnetically between the chains around T-N approximate to 22 K. The spin exchanges calculated by DFT + U and fitted by Monte Carlo simulations allow for the quantification of an effective "overall" model. The specific role of the mu(3)-O bridge on the ferromagnetic (FM) exchanges is discussed, together with spin reorientations observed in the ordered state. Magnetocrystalline anisotropy along the [110] direction stabilized by similar to 50 meV per Mn by spin-orbit coupling (SOC) was found by DFT + U + SOC.