  Attaining milli-Kelvin temperatures is often a prerequisite for the study of
novel quantum phenomena and the operation of quantum devices. Adiabatic
demagnetization refrigeration (ADR) is an effective, easy and sustainable
alternative to evaporation or dilution cooling with the rare and
super-expensive $^3$He. Paramagnetic salts, traditionally used for mK-ADR,
suffer from chemical instability related to water of crystallization. We report
synthesis, characterization as well as low-temperature magnetization and
specific heat measurements of two new UHV compatible candidate materials
NaYbP$_2$O$_7$ and KYbP$_2$O$_7$. Utilizing the PPMS at 2 K, the ADR of
sintered pellets with Ag powder admixture starting at 5 T yields base
temperatures (warm-up times) of 45 mK (55 min) and 37 mK (35 min) for
NaYbP$_2$O$_7$ and KYbP$_2$O$_7$, respectively, slightly advantageous to
KBaYb(BO$_3$)$_2$ (45 mK and 40 min) studied under similar conditions.
