| Summary: | In this study, we theoretically investigate the structural, electronic and magnetic properties of the Cu<sub>2</sub>OX<sub>2</sub> (X = Cl, Br, I) compounds. Previous studies reported potential spin-driven ferroelectricity in Cu<sub>2</sub>OCl<sub>2</sub>, originating from a non-collinear magnetic phase existing below <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>N</mi></msub></semantics></math></inline-formula>∼70 K. However, the nature of this low-temperature magnetic phase is still under debate. Here, we focus on the calculation of <i>J</i> exchange couplings and enhance knowledge in the field by (i) characterizing the low-temperature magnetic order for Cu<sub>2</sub>OCl<sub>2</sub> and (ii) evaluating the impact of the chemical pressure on the magnetic interactions, which leads us to consider the two new phases Cu<sub>2</sub>OBr<sub>2</sub> and Cu<sub>2</sub>OI<sub>2</sub>. Our <i>ab initio</i> simulations notably demonstrate the coexistence of strong antiferromagnetic and ferromagnetic interactions, leading to spin frustration. The <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>N</mi></msub></semantics></math></inline-formula> Néel temperatures were estimated on the basis of a quasi-1D AFM model using the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mrow><mi mathvariant="italic">ab</mi><mi mathvariant="italic">initio</mi></mrow></mrow></semantics></math></inline-formula><i>J</i> couplings. It nicely reproduces the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>N</mi></msub></semantics></math></inline-formula> value for Cu<sub>2</sub>OCl<sub>2</sub> and allows us to predict an increase of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>N</mi></msub></semantics></math></inline-formula> under chemical pressure, with <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>T</mi><mi>N</mi></msub></semantics></math></inline-formula> = 120 K for the dynamically stable phase Cu<sub>2</sub>OBr<sub>2</sub>. This investigation suggests that chemical pressure is an effective key factor to open the door of room-temperature multiferroicity.
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