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The crystal structure of magnetoelectric KNiPO4 was refined by single crystal X-ray diffraction methods at room temperature confirming orthorhombic space group Pna21. Powder neutron diffraction investigations showed that the room temperature structure is preserved down to 30 K. Consistent with magnetic susceptibility measurements, long-range antiferromagnetic ordering according to K = 0 and Shubnikov space group Pna21 is established by neutron diffraction at temperatures below TN = (25.5 ± 0.5) K. The ordered magnetic moments of (3.00 ± 0.04) μg, oriented along [1, 0, 0], indicate large orbital contributions to the magnetic moment of Ni2+. DTA studies in the temperature range from 100-1000°C, showed two phase transitions, at 486.5°C and at 581.3°C.

Crystal structures of rare-earth gallium oxide perovskites RGaO3, (R = La, Nd) have been studied by powder neutron diffraction and X-ray single crystal diffraction. The single crystals were grown by the Czochralski method. From power neutron diffraction we know that lanthanum gallium oxide, LaGaO3, is orthorhombic with space group Pbnm at room temperature and shows a first order phase transition at 425 K to a rhombohedral phase (R3c). The room temperature structure of neodymium gallium oxide, NdGaO3, has been determined by X-ray diffraction on a single domain crystal. The compound is orthorhombic with space group Pbnm. Polished samples with different thickness were studied by polarized light microscopy. The birefringence δn has been measured by orthoscopy and conoscopy versus wavelength and temperature. The orientation of the indicatrix has been correlated with the chemical unit cell. Further the absorption spectrum of Nd3+ in NdGaO3, at room temperature in the range of 400 nm ≤ Λ ≤ 800 nm was measured.