Inelastic neutron scattering and magnetic susceptibility measurements have been performed on the distorted perovskite NdGaO3. The magnetic susceptibility data show a Curie-Weiss behaviour with an effective magnetic moment close to 3.6 mu B per mole of Nd ions. No long-range magnetic ordering was detected in the temperature range 2-300 K. The inelastic neutron spectra observed at T=12 K exhibit four peaks of magnetic origin between 11 and 7D meV which can be unambiguously assigned to the complete crystalline-electric-field splitting pattern in the ground-state J multiplet 4I9/2 of the Nd3+ ions. We analysed the spectra in terms of geometrical considerations based on the actual C2 site symmetry of Nd3+. The best agreement between the experimental spectra and the calculated level structure was obtained for a model that takes into account the three nearest-neighbouring coordination polyhedra associated with the O2-, Ga3+ and Nd3+ ions as well as J-mixing between all multiplets of the 4I term. We conclude that single-particle crystal-field theory adequately explains the majority of magnetic and crystal-field properties of NdGaO3.

The rare-earth gallium oxide perovskites RGaO3 (R=La,Pr,Nd) are promising substrates for epitaxy of high-Tc superconductors. In this paper the crystal structures of orthorhombic and rhombohedral RGaO3 have been studied by neutron diffraction, which allows precise localization of light atoms such as oxygen, which are of importance in phase transitions. The lattice parameters, atom positions and tilt angles of the oxygen octahedra were determined in the temperature range 12-1773 K. Lanthanum gallate LaGaO3 shows a phase transition from an orthorhombic phase (Pbnm) to a rhombohedral phase (Rc) at 425 K. The results are discussed in the classification schemes developed for distorted perovskites.

The three-dimensional magnetic ordering of the Nd' ions in NdGa03 has been studied by means of neutron powder diffraction. The transition to the antiferromagnetic state takes place at T&=1 K.Below this temperature, the Nd magnetic moments are oriented along the c axis in a c, mode. The value of the saturated magnetic moment ( = 1.1p& ) is drastically reduced below the free ion value (3.27p& ) due to crystal-field effects. The main contribution to the Nd-Nd magnetic interaction appears to be superexchange, the classical dipole-dipole interaction being too small to account for the observed Neel temperature. Below 300 mK, we observe an additional enhancement of the magnetic reAections. The analysis of the Q dependence of the neutron powder-diff'raction intensities allowed us to interpret this increase as due to the polarization of the nuclear moments of the ' 'Nd and ' 'Nd isotopes in the hyperfine field created by the Nd + electronic moments. The particular conditions which lead to the observation of this phenomenon in NdGa03 are briefly discussed. I. INTRQDUCTIQNIt is well known that the hyperfine field created by the electronic magnetic moments can be used to orient the nuclear spins. ' Although several experimental methods can be used for the study of this phenomenon, the scattering of thermal neutrons is particularly appealing since its cross section depends on the relative orientation of the spins of the neutrons and the nuclei.Because of its high scattering power for this process, the ' Nd and ' Nd isotopes are ideal candidates for the study of nuclear polarization effects. In fact, in the recent literature, they have been already observed in several Nd-based compounds as NdPd3, Nd2Cu04, or NdNi03. The quantitative analysis of this phenomenon in the former cases was however rather complicated. In NdPd3, the reason was the incommensurate magnetic structure displayed by the Nd magnetic moments. For Nd2CuO4 and NdNi03, the presence of a second magnetic ion (Cu and Ni +, respectively) introduced also additional difficulties: in NdNi03, for example, a nuclear polarization of 60% and an increase of the electronic magnetic moment of Nd + from 2p~t o = 3pz produce Rietveld refinements with identical reliability factors. To avoid these difficulties, we decided to study the perovskite NdGa03, with only one type of magnetic ion (Nd +). The crystallographic structure of NdGa03 is nowadays well known. In a previous paper dealing with the rare-earth gallates RGa03 (R=La,pr, Nd), we reported several neutron powder-diffraction experiments showing that the three compounds are orthorhombically distorted perovskites between room temperature and 12 K. In the case of LaGaO3 and PrGa03, the collective tilts of the Ga06 octahedra correspond to those of the GdFe03-type structure, which is described by the centrosymmetric Pbnm space group. On the other hand, NdGa03 was described by the noncentrosymmetric Pbn 2, space group, following the single-crystal x-ray diffraction study of Brusset. Recent higher-resolutionx-ray diffraction exp...

Inelastic neutron scattering has been employed to study the erystallineelecbiofield interaction of the distorted perowkite ErCaO,. The observed energy spectra show several well defined inelastic peaks of magnetic origin in an energy transfer range up to 86 meV which are attributed to crystalline-electric-field transitions within the ground-state, J multiplet )H4 of the P I ' + ions. The spectra are analysed in terms of single-particle crystal-field theory based on geometrical coordinations associated with the C, site symmetry of the P?+ ions. J mixing as well as the imaginq part of the crystal-field Hamiltonian are found to be important to reproduce the experimental data. The resulting crystal-field parameters are close to the values extrapolated from a similar study of NdGa03. Our approach turns out to constitute a useful extrapolation scheme for other structumlly related perovskitetype compounds.