Synthesis and thermal behavior of KCe$_2$(PO$_4$)$_3$, a new full-member in the A$^{\mathrm{I}}$M$^{\mathrm{IV}}_2$(PO$_4$)$_3$ family

Hydrothermal treatment of nanoscale amorphous ceric phosphate gel in KOH aqueous solutions was found to result in a new KCe$_2$(PO$_4$)$_3$ phase. The refinement of the KCe$_2$(PO$_4$)$_3$ structure showed that it was isostructural to recently reported (NH$_4$)Ce$_2$(PO$_4$)$_3$. For the KCe$_2$(PO$_4$)$_3$ phase, the unit cell parameters (sp. gr. $C2/c$) were $a$ = 17.3781(3)$\mathring{\mathrm{A}}$, $b$ = 6.7287(1)$\mathring{\mathrm{A}}$, $c$ = 7.9711(2)$\mathring{\mathrm{A}}$, $\beta$= 102.351(1)$^\circ$, $V$ = 910.53(4)$\mathring{\mathrm{A}}^3$, $Z$ = 4. The thermal decomposition of KCe$_2$(PO$_4$)$_3$ at 800$^\circ$C resulted in the mixture of crystalline CePO$_4$ and KPO$_3$.