Strontium cobaltite (SrCoO_2.5+δ , SCO) is a fascinating material because of its topotactic structural phase transition caused by a change in oxygen stoichiometry. In the brownmillerite phase (δ=0) it is an insulating antiferromagnet whereas in the perovskite phase (δ=0.5) it is a conducting ferromagnet. In contrast, the impact of the varying Co/Sr stoichiometry on the structure has not yet been studied in SCO thin films. Using molecular beam epitaxy we have fabricated SCO thin films of varying Co/Sr stoichiometry. Films with Co excess exhibit a brownmillerite crystal structure with CoO precipitates within the thin film and on the surface. Co deficient films are amorphous. Only for 1:1 stoichiometry a pure brownmillerite structure is present. We find a clear dependence of the Reflection High Energy Electron Diffraction (RHEED) pattern of these thin films on the stoichiometry. Interestingly, RHEED is very sensitive to a Co excess of less than 12% while x-ray diffraction fails to reveal that difference. Hence, using RHEED, the stoichiometry of SCO can be evaluated and tuned in-situ to a high degree of precision, which allows for a quick adjustment of the growth parameters during a sample series.

钴酸锶（SrCoO 2.5 _{+ δ} ，SCO）是一种引人入胜的材料，因为它是由氧化学计量变化引起的全能结构相变。在棕褐铁矿相（δ = 0）中，它是一个绝缘的反铁磁体，而在钙钛矿相中（δ= 0.5）它是一个导电铁磁体。相反，尚未在SCO薄膜中研究Co / Sr化学计量比的变化对结构的影响。使用分子束外延，我们制造了具有不同Co / Sr化学计量比的SCO薄膜。具有过量Co的膜表现出棕褐色的晶体结构，并且CoO在薄膜内和表面上沉淀。缺钴膜是无定形的。仅对于化学计量比为1：1的情况，才会存在纯的褐镁橄榄石结构。我们发现这些薄膜的反射高能电子衍射（RHEED）模式与化学计量关系明显。有趣的是，RHEED对小于12％的Co过量非常敏感，而X射线衍射无法揭示这种差异。因此，使用RHEED，可以对SCO的化学计量进行评估和原位调整 高度精确，可以快速调整样品系列中的生长参数。