A materials-science student examines purified samples labeled \(P\), \(Q\), and \(R\). The samples are \(\text{SiC}\), \(\text{W}\), and \(\text{MgF}_2\), represented without repetition. The student records the following observations.

Sample	Electrical conductivity as a solid at \(25^\circ\text{C}\)	Response to applied stress	Observation during strong heating at \(1\ \text{atm}\)
\(P\)	Very low	Fractures and is very difficult to scratch	Decomposes before a stable liquid is obtained
\(Q\)	High	Can be rolled into a thin sheet after annealing	Forms an electrically conducting liquid
\(R\)	Very low	Cleaves when struck	Forms an electrically conducting liquid

Which assignment and particle-level explanation are most consistent with all the observations?