We describe a systematic study of the hydrothermal synthesis of rare-earth orthochromites, RCrO3 (R = La, Pr, Sm, Gd, Dy, Ho, Yb, and Lu) and YCrO3. All nine of these materials can be prepared in a single step by hydrothermal treatment of an amorphous mixed-metal hydroxide at temperatures above 300 °C upon heating around 24 h, with no post-synthesis annealing needed. The as-made materials are highly crystalline powders with submicrometer particle size. In the case of LaCrO3 the addition of solution additives to the hydrothermal synthesis allows some modification of crystallite form of the material, and in the presence of sodium dodecylsulfate nanocrystalline powders are produced. Profile refinement of powder X-ray diffraction (XRD) data show that each of the RCrO3 materials adopts an orthorhombic distorted (Pbmn) perovskite structure. Detailed, magnetization studies as a function of temperature reveal the high quality of the specimens, with low temperature antiferromagnetic behavior seen by direct current...

Direct Hydrothermal Synthesis and Physical Properties of Rare-Earth and Yttrium Orthochromite Perovskites

Raman study on a select few orthochromites, RCrO$_3$ (R = Y, Lu, Gd, Eu and Sm) shows that the phonon behavior at TN in compounds with magnetic R-ion (Gd and Sm) is remarkably different from that of non-magnetic R-ion (Y, Lu and Eu). While anomalies in most of the observed phonon frequencies in all these compounds may result from the distortion of CrO$_6$ octahedra due to size effect and magnetostriction arising from Cr-ordering, the anomalous behavior of their linewidths observed at TN for the compounds with only magnetic R-ion suggests spin-phonon coupling. The presence of spin-phonon coupling and the anomalies in the low frequency modes related to R-ion motion in orthochromites (R = Gd and Sm) support the suggestion that the coupling between 4f-3d moments play important role in inducing switchable electric polarization.

Spin-phonon coupling in multiferroic RCrO$_3$ (R-Y, Lu, Gd, Eu, Sm): A Raman study

Raman study on a select few orthochromites, RCrO3 (R = Y, Lu, Gd, Eu and Sm), shows that the phonon behavior at TN in compounds with magnetic R-ion (Gd and Sm) is remarkably different from that of non-magnetic R-ion (Y, Lu and Eu). While anomalies in most of the observed phonon frequencies in all these compounds may result from the distortion of CrO6 octahedra due to size effect and magnetostriction arising from Cr-ordering, the anomalous behavior of their linewidths observed at TN for the compounds with only magnetic R-ion suggests spin-phonon coupling. The presence of spin-phonon coupling and the anomalies in the low-frequency modes related to R-ion motion in orthochromites (R = Gd and Sm) support the suggestion that the coupling between 4f-3d moments play an important role in inducing switchable electric polarization.

Spin-phonon coupling in multiferroic RCrO3 (R-Y, Lu, Gd, Eu, Sm): A Raman study

MgSiO 3 in the perovskite structure, thought to be the dominant mineral phase of the earth's mantle, has been studied by both zero-pressure Raman spectroscopy and by infrared spectroscopy to pressures of 27 GPa. A total of eight bands are observed: four in the Raman spectrum between 250 and 500 cm -i, and four in the mid-infrared between 500 and 800 cm -1. These bands are divisible into three separate groupings: high frequency (-800 cm-1), caused by asymmetric silicon-oxygen stretching motions; intermediate frequency (370--690 cm-1), modes predominantly due to octahedral stretching and bending vibrations; and low frequency (200--300 cm-1), involving Mg 2+ motion. The pressure shift of the four mid-infrared modes yields an average mode Grtineisen parameter of 1.36 + 0.15. Systematic trends observed between mode Grtineisen parameters and mode frequency are used to derive an estimate of the thermodynamic Grtineisen parameter from the spectroscopic data. In this way, we derive an estimated value of -1.9, in good agreement with the thermodynamic value of 1.77 measured at zero pressure.

Vibrational spectrum of MgSiO3 perovskite: Zero‐pressure Raman and mid‐infrared spectra to 27 GPa

The rare earth orthochromites are extremely interesting due to the richness of their optical, dielectric, and magnetic properties as well as due to their multiferroic properties which make them suitable materials to study in the nanoregime. However, the wet-chemical synthesis of these materials in nanosize is nontrivial. Here, we report for the first time, the detailed Raman spectra as well as magnetic and dielectric properties of chemically synthesized GdCrO3 nanoparticles of size ranging from 40 to 60 nm. The magnetic properties are dictated by competing Cr3+–Cr3+, Gd3+–Cr3+, and Gd3+–Gd3+ superexchange interactions in different temperature regions, resulting into an antiferromagnetic ordering at 167 K due to the Cr3+–Cr3+ followed by weak ferromagnetic ordering due to the onset of Cr3+–Gd3+ interactions. At lower temperature, it shows weak antiferromagnetic ordering due to Gd3+–Gd3+ interaction. Below 95 K, GdCrO3 nanoparticles showed the presence of negative magnetization due to Gd3+ and Cr3+ interact...

K. Tsushima

Papers

Influence of magnetic ordering on the phonon Raman spectra in YCrO3 and GdCrO3