Microtexture of c-Axis-Oriented Polycrystalline Lanthanum Silicate Oxyapatite Formed by Reactive Diffusion

We annealed the La2SiO5/La2Si2O7 diffusion couple at 1873 K for 25 h to prepare the grain‐oriented lanthanum silicate oxyapatite (LSO) polycrystal at the interfacial contact boundary. The polycrystalline LSO was characterized using optical microscopy, transmission electron microscopy, scanning electron microscopy, and X‐ray diffractometry (XRD). The external shape line information on individual crystal grains were extracted from the backscattered electron image on the section surface of the polycrystal. We collected the electron backscatter diffraction patterns and XRD patterns to determine, respectively, the crystallographic orientations of individual grains and orientation degrees of the LSO polycrystal. The reaction area was in the form of a layer parallel to the original contact boundary, and consequently categorized, based on the differences in grain size distribution and orientation degree, into three distinct regions. The innermost region with the layer thickness of 30 μm was composed mainly of the rounded and relatively small crystal grains with the size less than ~16 μm in diameter. Furthermore, this region showed the lowest orientation degree among the three regions. On the other hand, the LSO grains on both sides of this region were much larger in size and prismatic in morphology; the LSO crystals were elongated along the c‐axis, and built up of faces {001}, {304}, and {100}. The individual crystal grains were aligned almost along their c‐axes, with their a‐axis directions being randomly distributed around the common c‐axis. The differences in microtexture among the three regions would be induced by the distinct growth behaviors of LSO crystals.