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Given the significant impact of fO2 on material properties, it is important to perform studies at fO2 values relevant to the sample of interest. However, current systems used to control fO2 in solid media assemblies (e.g., sliding sensors, graphite capsule buffering) have limited ability to produce the wide fO2 ranges necessary to simulate more extreme planetary interiors. NASA's fO2 control system implements a modified double capsule design that utilizes a wide range of solid metal-oxide buffers to control fO2 across a wide range of conditions. The approach is a modification of the previous double capsule design wherein the outer capsule itself acts as the metal component of the buffer assemblage, and the inner capsule to which the astromaterial sample of interest is placed resides within the outer capsule. To achieve higher experimental temperatures above the melting point of more traditional noble metal capsule materials, outer capsules of the control assembly are comprised of refractory (high melting point) metals such as Ni, Co, W, Fe, Mo, V, Cr, Nb and Ta. Resultant experiments have reliably achieved temperatures exceeding 1600 degrees C, and when used in conjunction with a multi-anvil press, system pressures of over 20 GPa can be obtained lending researchers the ability to simulate on or off-world geologic conditions previously difficult to obtain in a laboratory. NASA's fO2 control system was developed to enable high pressure, high temperature experimental studies of astromaterials at fO2 values relevant to the sample of interest. However, it may also be useful for the synthesis of materials where fO2 control is required (e.g., synthesis of crystal structures that might be stable under higher oxygen pressure). Further use cases may include mineral or melt syntheses, metal-silicate or mineral-melt element partitioning, phase equilibria studies, and the possible development of new chemical and mineral compounds that could not be manufactured in laboratories before. This fO2 control system technology is at a technology readiness level (TRL) 6 (system/subsystem prototype demonstration in a relevant environment). The innovation is now available for your company to license. Please note that NASA does not manufacture products itself for commercial sale.