https://research.ibm.com/publications/glasslike-thermal-properties-and-isotope-effect-in-rb1-xnh4xh2po4-mixed-crystals
Glasslike thermal properties and isotope effect in Rb1-x(NH4)xH2PO4 mixed crystals for Physical...
Glasslike thermal properties and isotope effect in Rb1-x(NH4)xH2PO4 mixed crystals for Physical Review Letters by J.-F. Berret et al.