为什么氧化锌加热变成黄色？

一种说法阳离子极化能力增大，共价键极化性增大，颜色加深
还有一种说法，原文如下
ZnO occurs as white powder known as zinc white or as the mineral zincite. The mineral usually contains a certain amount of manganese and other elements and is of yellow to red color.[2] Crystalline zinc oxide is thermochromic, changing from white to yellow when heated and in air reverting to white on cooling.[3] This is caused by a very small loss of oxygen at high temperatures to form the non-stoichiometric Zn1+xO, where at 800 °C, x= 0.00007.[3]