Crystal and magnetic structure of the La${}_{1-x}$Ca${}_x$MnO${}_3$ compound $(x=0.8,0.85)$

We studied the crystal and magnetic structure of the La${}_{1-x}$Ca${}_x$MnO${}_3$ compound for $x=0.8$ and $x=0.85$. At T=300 K both samples are paramagnetic with crystallographic symmetry $Pnma$. At low temperatures they undergo a monoclinic distortion from orthorhombic $Pnma$-type structure with $a_p\sqrt{2}\times 2a_p\times a_p\sqrt{2}$ to a monoclinic structure with ($a_p\sqrt{2}\times 2a_p\times a_p\sqrt{2}$, $\beta =90+ϵ\sim {91.4}^{\mathrm{o}}$) and $P{2}_1\mathrm{/}m$ space group below $T_N$. The onset of the structural transformation coincides with the development of the $C$-type long range antiferromagnetic order with propagation vector ${\bf k}=({1/2},0,{1/2})$. The monoclinic unit cell allowed us to determine the direction of the Mn magnetic moment with respect to the crystallographic axes: it is perpendicular to the propagation vector, ${\bf m}\perp {\bf k}=({1/2},0,{1/2})$\mathrm{.}Theamplitudeoftheorderedmagneticmomentat$T=1.6$ K is found to be $2.53(2)$ and $2.47(2){\mu }_B$ for $x=0.8$ and 0.85, respectively.