| Summary: | The mixed homonuclear and heteronuclear hydrogen bonds in ammonia hydrates have been of interest for several decades. In this manuscript, a neutron powder diffraction study is presented to investigate the structure of ammonia monohydrate IV at 170 K at an elevated pressure of 3–5 GPa. The most plausible structure that accounts for all features in the experimental pattern was found in the P<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mn>2</mn><mn>1</mn></msub><mo>/</mo><mi>c</mi></mrow></semantics></math></inline-formula> space group and has the lattice parameters <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>a</mi><mo>=</mo><mn>5.487</mn><mo>(</mo><mn>3</mn><mo>)</mo></mrow></semantics></math></inline-formula> Å, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>b</mi><mo>=</mo><mn>19.068</mn><mo>(</mo><mn>4</mn><mo>)</mo></mrow></semantics></math></inline-formula> Å, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>c</mi><mo>=</mo><mn>5.989</mn><mo>(</mo><mn>3</mn><mo>)</mo></mrow></semantics></math></inline-formula> Å, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>β</mi><mo>=</mo><mn>99.537</mn><mo>(</mo><mn>16</mn><mo>)</mo></mrow></semantics></math></inline-formula> deg. While the data quality limits discussion to a proton-ordered structure, the structure presented here sheds light on an important part of the ammonia–water phase diagram.
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