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We report two new rare-earth (R) ternary intermetallic compounds—Ho2Ni0.8T1.2 with T = Si and Ge—that correspond to the R5Ni2T3 phase earlier reported to form in Dy–Ni–T and Ho–Ni–T ternary systems. The compounds crystallize in a filled version of the orthorhombic Zr2Ni1–xP-type structure with x = 0.52; their stoichiometry, determined from both single-crystal and powder X-ray diffraction data, is centered on Ho2Ni0.8T1.2 with a narrow solid solubility range for the silicide, while the germanide appears to be a line phase. In addition to R = Dy and Ho, R2Ni0.8T1.2 compounds also form for R = Y and Tb, representing the first examples of rare-earth-based compounds adopting the Zr2Ni1–xP structural prototype. Bulk magnetization data reveal the main transitions of the ferrimagnetic or ferromagnetic type at TC = 38 K for Ho2Ni0.8Si1.2 and TC = 37 K for Ho2Ni0.8Ge1.2, which are followed by subsequent magnetic reordering at lower temperatures. Neutron diffraction shows complex magnetic structures below TC with both ferromagnetic and antiferromagnetic components and magnetic propagation vector κ1 = [0, 0, 0]. Below TN ≅ 24 K (22 K) for the silicide (germanide), an additional antiferromagnetic coupling following an incommensurate magnetic propagation vector κ2 = [κx, 0, 0] appears to coexist with the first magnetic structure., The crystal structure of Ho2Ni0.8Si1.2 (and Ho2Ni0.8Ge1.2) [Zr2Ni0.48P-type (oP32-Pnma)] viewed along the b axis is given. The Ho, Ni1, Ni2/Si, Si1 and Si2 atoms are represented as red, turquoise, blue, light-gray, and dark-gray spheres, respectively. The distinctive structural fragment is built up of five differently oriented trigonal prisms. The magnetic structure of Ho2Ni0.8Ge1.2 with the antiferromagnetic component in the c direction and the ferromagnetic component along the a axis.

Magnetic frustration results from competing magnetic interactions and leads to unusual ground states, ranging from non-collinear magnetic orders to spin liquids (SLs), opening the path to new physics and emerging properties. We report the engineered magnetic interaction design and synthesis of the new ternary nitride MnTa2N4 with a normal spinel structure, where the magnetic cations Mn2+ occupy exclusively the tetrahedral sites forming a diamond lattice. Although the magnetic interactions are strongly antiferromagnetic (θCW ≈ -140 K), a long-range magnetic order is not established, but a smooth magnetic anomaly is observed at T∗ ≈ 5.1 K, fingerprinting a large magnetic frustration. A short-range helicoidal magnetic order emerges at low temperatures. The ordered moment is ≈70% of the expected magnetic moment of Mn2+ ions and a large part (≈28%) of the spin entropy remains at 400 mK, signaling the coexistence of a helicoidal order with spin-glass-like or SL textures. First-principles calculations unveil an unexpected large direct Mn-Mn exchange interaction that balances the superexchange and accounts for the magnetic frustration. These findings open new avenues toward the design of quantum materials., This work was supported by the Ministerio de Ciencia e Innovación, Spain (PID2020-113805GB-I00, PID2020-118479RB-I00 (AEI/FEDER, EU), PGC2018-096955-B-C44 ,PGC2018-093863-B-C22, CEX2019-000917-S and MDM-2017-0767) and from Generalitat de Catalunya (2017SGR1377, 2017SGR581, 2017SGR1506, and 2017SGR1289). We thank Alba and ILL for beam time provision (experiments 2019023454 and 5-31-2713, respectively), Dr. François Fauth for assistance in data collection at Alba, and Dr. Bernat Bozzo (ICMAB-CSIC) for performing magnetic measurements. The authors acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

We present the synthesis of a novel binary metal oxide material: Ba7Mn4O15. The crystal structure has been investigated by high-resolution powder synchrotron X-ray diffraction in the temperature range of 100–300 K as well as by powder neutron diffraction at 10 and 80 K. This material represents an isostructural barium-substituted analogue of the layered material Sr7Mn4O15 that forms its own structural class. However, we find that Ba7Mn4O15 adopts a distinct magnetic ordering, resulting in a magnetoelectric ground state below 50 K. The likely magnetoelectric coupling mechanisms have been inferred from performing a careful symmetry-adapted refinement against the powder neutron diffraction experiments, as well as by making a comparison with the nonmagnetoelectric ground state of Sr7Mn4O15.

Pyrophosphates of the general formula A2BP2O7 (A = alkali metal and B = transition metal) find applications in several technological fields, including rechargeable Na-ion batteries. They present a rich polymorphism vs composition, which may strongly influence their functional properties. Therefore, to selectively obtain a specific structure is crucial for applications. Here, we combine structural investigation techniques (solid-state NMR, X-ray and neutron diffraction, and DFT calculations) to investigate the crystal chemistry of the series Na2(M/M’)P2O7 with couples M/M’ = Co/Zn and Co/Fe. For both the series, a phase transition vs composition is observed. A tetragonal to orthorhombic phase transition is found for the Zn/Co series, and an orthorhombic to triclinic phase transition is found for the Co/Fe series. Such changes are interpreted in view of the different electronic structures of the transition metal ion. In addition, with the support of modeling, short- and long-range structural analysis, we show that the coexistence of two polymorphs for a given composition is possible, suggesting that the final structures may be strongly dependent upon the synthesis procedure.

The development of new synthetic methodologies of perovskite oxynitrides is challenging but necessary for the search of new compounds and the investigation of new properties. Here, we report a new method of preparation of the perovskite LaTaON2 that has been investigated as a pigment and photocatalyst for water splitting. The synthesis proceeds through the solid-state reactions under N2 at 1500 °C between La2O3, LaN, and Ta3N5 or between LaN and TaON, which are completed after 3 h and lead to sintered, highly crystalline samples with particle sizes up to 1 μm. Nitrogen-deficient samples LaTaO1+xN2-x with x ≤ 0.35 are prepared by changing the N/O ratio in the mixture of reactants. Electron diffraction, synchrotron diffraction, and neutron diffraction studies on stoichiometric and nitrogen-deficient compounds indicate that they crystallize in the monoclinic space group I2/m with lattice parameters for LaTaON2 of a = 5.71458(7), b = 8.05987(10), c = 5.74772(6) Å, and β = 89.982(3)°. The three anion sites of the I2/m structure are partially occupied by oxygen and nitrogen, with a preference of nitride for two positions with occupancies of 77 and 88%. This anion distribution is different from that reported in previous studies of samples prepared by ammonolysis at lower temperature, suggesting that the synthesis conditions affect the anion order of this perovskite. Optical measurements indicate a band gap of about 1.9 eV, which is close to that observed in samples prepared by other methods. The determined dielectric permittivity for LaTaON2 εr ≈ 200, reported for the first time for a highly nitrided pseudocubic perovskite, is similar to that observed in perovskites with one nitrogen per formula., This work was supported by the Ministerio de Ciencia e Innovación, Spain [MAT2017-86616-R, MAT2017-85232-R, PID2019-107727RB-I00, PID2020-113805GB-I00, PID2020-118479RB-I00, and CEX2019-000917-S], from Generalitat de Catalunya (2017SGR1377 and 2017SGR581) and from CSIC through the i-LINK (LINKA20338) program. The authors thank Alba and ILL for beam time provision (experiments 2017092391 and 5-21-1113, respectively), Dr. François Fauth for assistance in data collection at Alba, and Dr. Bernat Bozzo (ICMAB-CSIC) for performing electrical measurements. The project was also supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. I.F. acknowledges the Ramón y Cajal contract RYC-2017-22531 and JRG acknowledges the AEAT predoctoral fellowship PRE2018-085204., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Room-temperature multiferroism in polycrystalline antiferromagnetic Fe perovskites is reported for the first time. In the perovskite-type oxides RE1.2Ba1.2Ca0.6Fe3O8 (RE = Gd, Tb), the interplay of layered ordering of Gd(Tb), Ba, and Ca atoms with the ordering of FeO4-tetrahedra (T) and FeO6-octahedra (O) results in a polar crystal structure. The layered structure consists of the stacking sequence of RE/Ca-RE/Ca-Ba-RE/Ca layers in combination with the TOOT sequence in a unit cell. A polar moment of 33.0 μC/cm2 for the Gd-oxide (23.2 μC/cm2 for the Tb one) is determined from the displacements of the cations, mainly Fe, and oxygen atoms along the b-axis. These oxides present antiferromagnetic ordering doubling the c-axis, and the magnetic structure in the Tb compound remains up to 690 K, which is one of the highest transition temperatures reported in Fe perovskites.

The effect of substituting iron and zinc for cobalt in CaBaCo4O7 was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCo4O7 decreases with increasing the content of either Fe or Zn. The samples CaBaCo3ZnO7 and CaBaCo4–xFexO7 with x ≥ 1.5 are metrically hexagonal, but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe suppresses the ferrimagnetic ordering of the parent compound, and CaBaCo4–xFexO7 (0.5 ≤ x ≤ 2) samples are antiferromagnetically ordered following a new propagation vector k = (1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long-range magnetic interactions giving rise to a magnetic glass-like behavior in the CaBaCo3ZnO7 sample. The oxidation states of iron and zinc are found to be 3+ and 2+, respectively, independently of the content, as confirmed by X-ray absorption spectroscopy. Therefore, the average Co oxidation state changes accordingly with the Fe3+ or Zn2+ doping. Also, X-ray absorption spectroscopy data confirm the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and X-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co2+/Co3+ valence state or, alternatively, Co2+ and Co3+ ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO4 tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder are observed only at the ferrimagnetic transition for CaBaCo4O7, revealing the occurrence of local magneto-elastic coupling., The authors would like to acknowledge ILL and D2B and D1BCRG (MINECO) stations for beam time allocation (experiment code 5-31-2366, doi:10.5291/ILL-DATA.5-31-2366). We also acknowledge the ESRF for granting beam time, BM23 staff for technical assistance, and Olivier Mathon and Sakura Pascarelli for their support on the XAS experiments. Authors also acknowledge the use of Servicio General de Apoyo a la Investigacion-SAI, Universidad de Zaragoza. For the financial support, we thank the Spanish Ministerio de Economia y Competitividad (MINECO), Project MAT2015-68760-C2-1-P, and Diputacion General de Aragon (DGA), Project E69.