High temperature phases with wurtzite-derived structure in Zn₂LiGaO₄–ZnO alloy system

We investigated the structural phase transformations of Zn₂LiGaO₄, which is a quaternary oxide semiconductor with a wide band gap of 4.0 eV, and its alloys with ZnO by using in-situ high-temperature X-ray diffraction (XRD), room-temperature XRD and selected-area electron diffraction of samples quenched from high temperatures of >873 K. Three types of phases with the wurtzite-derived structure were found in the y(Zn₂LiGaO₄)_1/4–(1−y)ZnO alloy system. First is the low-temperature phase, phase I, in which constituent cations are ordered similarly to those in a Cd₂AgGaS₄-type structure. Phase I has an incommensurate modulation vector of 1.04b* and is stable at <1070 K and compositions of 0.96 ≤ y ≤ 1 and is stable at room temperature for y = 0.92. The second phase, phase II, appears at intermediate temperatures of 773–1373 K at 0.96 ≤ y ≤ 1, and its incommensurate modulation vector is 1.08b*. Phase III, which has no long-range cation ordering but still has short-range cation ordering, is stable for 0.92 ≤ y ≤ 1 at high temperatures and for 0.2 ≤ y < 0.92 from room temperature to high temperatures. We discuss the driving force of the transformations among the three phases in terms of their crystal structure and the mismatch in thermal expansion between –Zn–O–Zn–O– and –Li–O–Ga–O– linkages.