Highest melting pure oxide (> 3000 °C): which oxide among the following has the highest melting point and is used in extreme-temperature refractory applications?

Introduction / Context:
Melting point is a key indicator for selecting refractories for ultra-high temperature service. Pure oxides such as thoria, zirconia, alumina, and beryllia differ substantially in melting temperatures, thermal conductivities, and chemical stabilities.

Given Data / Assumptions:

• We focus on pure oxides and their intrinsic melting points.
• “Highest” implies comparing representative literature values.

Concept / Approach:
Thoria (ThO2) exhibits one of the highest melting points among common refractory oxides, generally quoted above 3000 °C (around 3300 °C). Zirconia and beryllia melt in the mid-to-upper 2000 °C range, and alumina near ~2050 °C. Hence ThO2 tops this list for extreme temperature stability.

Step-by-Step Solution:

Verification / Alternative check:
High-temperature materials references consistently list thoria at the top among these oxides; it has been used historically in crucibles and special electrodes where temperature extremes are encountered.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing service temperature with melting point; ignoring destabilization by impurities or atmosphere.

Final Answer:
Thoria (ThO2)