Identify the formula of compound $\left(\mathrm{B}\right)$ formed in following reaction.

(i) Lanthanoide $\left(\mathrm{Ln}\right)$ burnt in oxygen forming compound $\left(\mathrm{A}\right)$

(ii) Compound $\left(\mathrm{A}\right)$ reacts with ${\mathrm{CO}}_2$ in excess to form compound $\left(\mathrm{B}\right)$