New hypodiphosphates of the alkali metals: synthesis, crystal structure and vibrational spectra of the hypodiphosphates(IV) 'M IND. 2'[('H IND.2''P IND.2''O IND.6')('H IND.4''P. IND.2''O IND.6')] (M='RB' and 'CS') - Wu, Peng; Wiegand, Thomas; Eckert, Hellmut; Gjikaj, Mimoza (Journal of Solid State Chemistry, 2012)