New geometrically frustrated magnets prepared by high pressure and high temperature *

High pressure high temperature (HPHT) synthesis is a unique and effective method to explore new materials by stabilizing new structural phase. In this talk, we applied HPHT to synthesize new geometrically frustrated magnets and therefore to study their exotic quantum magnetism. We will discuss two examples. One is the spin-1 triangular lattice antiferromagnet Ba3NiSb2O9. This system shows a 6HA phase with TN = 12 K at ambient pressure, but it is transformed to 6HB phase under HPHT to exhibit quantum spin liquid state. Another example is the pyrochlores R2Ge2O7 and R2Pt2O7, which does not exist at ambient pressure due to small ionic size of Ge4+ and Pt4+, but could be synthesized under HPHT. We also studied their frustrated magnetism.