R. Saez Puche

Magnetic properties of the semiconducting lanthanide cuprates Ln2CuO4 and their interpretation: evidence for a new series of planar copper antiferromagnets

Abstract The magnetic susceptibility of the semiconducting lanthanide cuprates Nd 2 CuO 4 , Pr 2 CuO 4 , Eu 2 CuO 4 , and Sm 2 CuO 4 has been measured in the range 4–300 K. Below 300 K, the Cu 2+ ions are ordered antiferromagnetically in the CuO 2 planes of these compounds, and the exchange interactions involving the Ln 3+ ions are relatively weak. The suceptibility of the Ln 3+ ions obeys the Curie-Weiss law at elevated temperatures, but deviations from this law occur at lower temperatures. An attempt is made to account for these deviations by fitting theoretical expressions for the susceptibility of isolated Ln 3+ ions under the influence of a cubic crystal field to the experimental data. Excellent agreement is obtained for Nd 3+ and Eu 3+ over the entire temperature range and for Pr 3+ and Sm 3+ at elevated temperatures. Deviations at lower temperatures for the latter two ions may be due to structural changes, exchange interactions involving the Ln 3+ ions, or possibly oxygen nonstoichiometry. The susceptibility parameters derived by fitting the theoretical expressions to the experimental data are also discussed. It is concluded that these compounds form an interesting new series of planar Cu 2+ -ion antiferromagnets.

Experimental and theoretical study of structural properties and phase transitions in YAsO 4 and YCrO 4

We have performed experimental and theoretical studies of the structural stability of YAsO4 and YCrO4 at high pressures. X-ray diffraction experiments together with ab initio total-energy and lattice-dynamics calculations have allowed us to completely characterize a pressure-induced structural phase transition from the zircon to the scheelite structure in both compounds. Furthermore, total-energy calculations have been performed to check the relative stabilities of different candidate structures at different pressures and allow us to propose for YAsO4 the zircon \rightarrow scheelite \rightarrow SrUO4-type sequence of structures. In this sequence, sixfold arsenic coordination is attained for the SrUO4-type structure above 32 GPa. The whole sequence of transitions is discussed in comparison with YVO4, YPO4, YNbO4, YMoO4, and YTaO4. Also a comparative discussion of lattice-dynamics properties is presented. The band-gap for YAsO4 and YCrO4 and band structure for YAsO4 are also reported. Finally, the room-temperature equation of state of different compounds is also obtained.

Magnetic and X-ray diffraction characterization of stoichiometric Pr2NiO4 and Nd2NiO4 oxides

Abstract We report the crystal structure and the magnetic properties of a new nearly stoichiometric brown phase of Pr2NiO4 and Nd2NiO4. The Rietveld refinement of the powder X-ray diffraction patterns at room temperature allows us to assign the crystal structure to orthorhombic (Bmab), for both compounds. The magnetic susceptibility data shows a clear antiferromagnetic ordering for temperatures lower than 300K. Also a noticiable increase appears in the susceptibility around 120K in both oxides, which is associated with the tilt of the NiO6 octahedra, as has been recently reported in La2NiO4.

Magnetic properties of metallic La2CuO4

Abstract X-ray, magnetic susceptibility, and electron paramagnetic resonance measurements have been made on La2CuO4. Magnetic measurements in the range 4.2 – 300 K are consistent with the metallic nature of La2CuO4, and the itinerant - electron susceptibility is estimated to be 239 × 10−6 emu mol−1. The high susceptibility and density of states indicate that the conduction band is narrow. The results are discussed in terms of previous models of the level structure of La2CuO4, and it is concluded that conduction occurs via a σ∗ x 2 − y 2 band. Comparison of La2CuO4 with semiconducting lanthanide cuprates suggests that this compound is on the verge of a semiconductor-to-metal transition.

Antiferromagnetism in the semiconducting lanthanide cuprates Ln2CuO4

Abstract The semiconducting lanthanide cuprates Pr 2 CuO 4 , Nd 2 CuO 4 , and Gd 2 CuO 4 have been investigated using X-ray diffraction, magnetic susceptibility, and electron paramagnetic resonance techniques. The susceptibility and resonance studies indicate that the Cu 2+ ions are ordered antiferromagnetically and that the exchange interactions involving the Ln 3+ ions are relatively weak. The deviations from the Curie-Weiss law in Pr 2 CuO 4 and Nd 2 CuO 4 are in qualitative agreement with the predicted behavior for isolated Ln 3+ ions under the influence of a crystal field. Hence these compounds provide interesting examples of planar Cu 2+ -ion antiferromagnets.

Synthesis, characterization and magnetic properties of LnBO3 (Ln ≡ Nd, Gd, Tb, Dy, Ho and Er)

Abstract A new method for the synthesis of some lanthanide orthoborates LnBO 3 (Ln ≡ Nd, Gd, Tb, Dy, Ho and Er) has been successfully used. The different orthoborates LnBO 3 were obtained by mixing metal hydroxides Ln(OH) 3 and orthoboric acid B(OH) 3 in stoichiometric amounts at low temperatures between 873 and 973 K and at pressures of between 10 −4 and 10 −5 mm Hg. The results seem to indicate that this method of synthesis can be applied to the isolation of metal borates at temperatures that are several hundreds of degrees below those reported in the literature. The characterization of these materials has been carried out by X-ray powder diffraction and IR spectrometry. The magnetic susceptibilities of these orthoborates have also been measured in the temperature range 4.2–300 K. From 50 to 300 K the data fit a Curie-Weiss behaviour and the Weiss constants are negative for all the compounds. Magnetic susceptibility shows a quasi-Curie-like behaviour for GdBO 3 .

CORRELATION BETWEEN STRUCTURAL AND VIBRATIONAL PROPERTIES OF THE LN3SB5O12-TYPE OXIDES

Abstract The vibrational spectra of a series of mixed oxides of the Ln3Sb5O12 (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) type were recorded and are discussed in relation with their structural characteristics. The influence of the Ln(III) cations on the internal Sb-O vibrations was specially analysed.

Phase transitions and magnetic behaviour of R1−xCa1+xCrO4 oxides (RY or Sm) (0⩽x⩽0.5)

Abstract The RCaCrO4 (RSm or Y) stoichiometric oxides crystallize with orthorhombic symmetry and space group Bmab. In the case of the SmCaCrO4 at 1073 K a structural phase transition of the type Bmab (orthorhombic) → I4/mmm (tetragonal) has been observed. Below room temperature at about 190 K a second Bmab→P42/ncm (tetragonal) phase transition has been detected from X-ray diffraction data. The magnetic properties of both SmCaCrO4 and YCaCrO4 reveal the existence of antiferromagnetic ordering above room temperature. The appearance of the ferromagnetic component at the temperature at which the low temperature phase transition takes place has been explained as the result of the canting of the antiferromagnetically ordered chromium sublattice. A field-induced magnetic transition has been observed at 50 K and 14 kOe for SmCaCrO4. The doping of the samarium sublattice, which increases the Ca Sm ratio from 1 to 1.5 to give mixed-valence compounds of the type Sm1−xCa1+xCrO4, yields very important changes in the structural properties.

Crystal growth, IR and Raman spectra, thermal decomposition and magnetic properties of KLn(CrO4)2 (Ln ≡ Pr, Nd, Sm)

Abstract The double chromates of formula KLn(CrO4)2 (Ln ≡ Pr, Nd, Sm) were prepared as single crystals by hydrothermal synthesis from the oxides Pr6O11, Nd2O3 and Sm2O3 respectively and K2Cr2O7 solution at 150 °C in sealed tubes. These compounds were also obtained as microcrystalline powders in the above-mentioned hydrothermal process and by solid reaction of the oxides Pr6O11, Nd2O3 and Sm2O3 with KNO3 at 450 °C. These materials are isostructural with KLa(CrO4)2 which crystallizes in the monoclinic system with space group P21/c and Z = 4. Crystallographic data, IR and Raman spectra, thermal decomposition and magnetic properties of these compounds were studied.

Crystallographic data, magnetic and vibrational properties of the series Ln(OH)CrO4 (Ln≡La,Pr,Nd)

Abstract The synthesis of two new rare-earth hydroxochromates, Pr(OH)CrO4 and Nd(OH)CrO4, is described. Their crystallographic data are reported and compared with those of the isostructural lanthanum compound. The vibrational (IR and Raman) spectra of the three compounds, as well as that of the deuterated analogue, La(OD)CrO4, are described and discussed. The magnetic behaviour, in the temperature range between 4.2 and 300 K, was also investigated.