ABSTRACT
Transition metal (TM) compounds and various rare earth-based compounds have been the workhorse of the strongly correlated community over the past ve to six decades, with very interesting properties being found as a consequence of closely coupled spin, charge, orbital, and lattice degrees of freedom [1-7]. P-band materials, with their completely lled bands, were never considered candidates until recent studies brought back alkali (A) metal oxides into the spotlight. Alkali metal oxides have been explored in the 1960s and 1970s by various groups. A detailed review of these materials in 1989 summarized the motivating factors that drove research on these materials. In the 1960s, Hesse et al. [9] concluded that the presence of catenated oxygen (O) species, such as ozonide (O3
−), hyperoxide (O2 −), and peroxide
( O2 2− ), in these ionic compounds led to investigations devoted to the chemical nature
of these compounds that had such unusual compositions in their structure. However, in the second period from the 1960s to the late 1980s, the availability of large single crystals led to investigations on crystal chemistry, phase transitions, and physical properties.
