Both tungsten and molybdenum cannot be used in air at temperatures above 400°C. Crucibles made of these metals are intended exclusively for operation in a vacuum or in a protective atmosphere.

There is no real alternative to platinum for melting alkalis, but you can use a graphite crucible for short-term heating, such as with a burner.

thank you sir for the answer. I tried with graphite, but considerable amount of melt was remained in the crucible during the transfering process of the melt. It causes an considerable error in my analysis process

This replacement of cucibles is absolutely impossible as in air at such high temperature this method will produce molybdates and tungstates from your metals