In1-xGa1+xO3(ZnO)0.5: Synthesis, structure and cation distribution

Abstract

Crystals of the quaternary compound In1-xGa1+xO3(ZnO)0.5 were grown by the optical floating-zone method using a mirror furnace with halogen lamps as heat source. X-ray diffraction (XRD) and electron diffraction yielded a hexagonal crystal system and the space group P63/mmc (No. 194). Structure refinement from single crystal XRD data at −90 °C (a ​= ​3.3137(1) Å; c ​= ​29.523(1) Å; wR2: 0.054) revealed a structure isotypic to YbFeO3(FeO)0.5 and consists of three different alternating layers: (i) layer of edge-sharing InO6 octahedra, (ii) a pair of two layers of corner-sharing (Ga/Zn)O4 tetrahedra corresponding to wurtzite structure type, and (iii) a layer of GaO5 trigonal-bipyramids (tbp). The polyhedra of layers (ii) and (iii) share corners with the InO6 octahedra, resulting in inverted orientations of the two (Ga/Zn)O4 layers in the wurtzite type region. Additionally, atomic resolution imaging in the electron microscope proves a perfect periodic stacking of the atomic layers. The analysis of individual crystals revealed an excess of Ga expressed by the composition range In1-xGa1+xO3(ZnO)0.5 with 0.20 ​≤ ​x ≤ 0.82, determined by energy dispersive X-ray spectroscopy (EDXS). Consequently, the octahedral site is occupied with In and Ga in a ratio of In:Ga ​= ​0.80:0.20 to 0.18:0.82, assuming that the tbp sites are only occupied by Ga. The transparent crystals as-received from floating-zone growth had a blueish color and became colorless after annealing at 1000 ​°C for 10 ​h in air. The as-prepared crystals are oxygen deficient and contain unpaired electrons (color centers) at vacant oxide sites proven by electron paramagnetic resonance with g-value of ca. 2.