Thermodynamics of charged Brans-Dicke AdS black holes

It is well known that in four dimensions, black hole solution of the Brans-Dicke-Maxwell equations is just the Reissner-Nordstrom solution with a constant scalar field. However, in n⩾4 dimensions, the solution is not yet the (n+1)-dimensional Reissner-Nordstrom solution and the scalar field is not a constant in general. In this Letter, by applying a conformal transformation to the dilaton gravity theory, we derive a class of black hole solutions in (n+1)-dimensional (n⩾4) Brans-Dicke-Maxwell theory in the background of anti-de Sitter universe. We obtain the conserved and thermodynamic quantities through the use of the Euclidean action method. We find a Smarr-type formula and perform a stability analysis in the canonical ensemble. We find that the solution is thermally stable for small α, while for large α the system has an unstable phase, where α is a coupling constant between the scalar and matter field.