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Herein, the synthesis, characterization and thermal behaviour of imidazole-based two copper-phosphate mixtures, A = Cu 2 (PO 4 )(OH)ÁCu(HPO 4 )(H 2 O)Á(C 3 H 4 N 2 ) 2 . 3.25 H 2 O and B = Cu 3 (PO 4 ) 2 H 2 OÁ(C 3 H 4 N 2 ) 3 (H 2 O)Á0.1(C 3 H 4 N 2 ).3.25 H 2 O are reported. The characterization was done by adopting various electro-analyticaltechniques such as elemental analysis, X-ray Powder Diffraction (XRD), Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG), Fourier Transform Infrared (FT-IR) Spectrometry, Absorption Spectrophotometry and Ultraviolet-Visible and near Infrared (UV-Vis-NIR). Differential Scanning Calorimetry performed with the heating rate 10 K/min from 297.96 to 770.46 K in normal atmosphere for both mixtures. DSC data indicated that both mixtures A and B are exhibiting exothermic property by their net specific heat capacities (C p ) −11.11 and −2.83 J/g K, respectively. Therefore, these complex mixtures can be utilized as heat dissipation materials. Both mixtures A and B undergo phase change in terms of hydrated phase to dehydrated phase up to 356 and 392 K, respectively. The specific heat capacity of Mixture A during hydration, C p = 2.54 J/g K, is higher than the tin, lead, stainless steel, glass and aluminium at their respective melting points. This mixture is also found better than the commercial product based on lithium ion battery in terms of specific heat capacity. From UV-Vis-NIR analysis, it is found that the mixtures A and B are showing semiconducting behaviour with band gaps 1.66 and 1.68 eV, respectively. The average crystallite sizes of these nano-complex mixtures are 35.29 and 30.94 nm and these were calculated using the Debye-Scherrer equation and Williamson-Hall method.

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Four different mixtures such as complex mixture A, [Co(H 2 O) 4 (HPO 4 )] (C 3 H 4 N 2 )(H 2 O) 2 Á 0.2 (C 3 H 4 N 2 ), complex mixture B, [Co(H 2 O) 6 ](H 2 PO 4 )(OH) (H 2 O) Á(C 3 H 4 N 2 ) 2 , mixture C, 0.34 Co(PO 3 ) 2 Á0.15(C 3 H 3 N 2 Na), and complex mixture D, 0.03Co(PO 3 ) 2 Á0.25(C 3 H 3 N 2 Na) Á0.1 [Co(H 2 O) 6 ]SO 4 .H 2 OÁ0.6 (C 3 H 4 N 2 ), were synthesized using ortho-phosphoric acid and meta-phosphoric acid with cobalt sulfate heptahydrate and imidazole in ambient conditions. These mixtures have been well characterized by different thermo-analytical techniques. Differential Scanning Calorimetry (DSC) has been carried out at different heating rates such as 5, 10, 20, and 30 K min −1 at atmospheric conditions. The specific heat capacity was measured at low temperatures up to 173 K for mixture C at a rate of 10 K min −1 . From the thermal analysis, it can be observed that the complex mixtures A and B prepared using phosphoric acid and mixture C prepared using meta-phosphoric acid behave as heat repellent material owing to their exothermic behavior at high temperature. The anomalous negative specific heat capacity exhibited by the mixtures is explained using partial molar quantities. Among all the four complex mixtures, the complex mixture A has been found to be the best heat repellent material.The complex mixture B can be used as heat storage material up to temperature $550 K due to its endothermic behavior. These phosphate materials can be used for longer period of time due to their stability and insolubility in aqueous media. The heat can be extracted from any system that is coated with these phosphate mixtures using high conductivity copper rod. K E Y W O R D SDSC, heat repellent material, negative heat capacity, partial molar quantities | INTRODUCTIONConsumption of energy has increased exponentially over the years in all aspects of human life. Because of this, the modern technological man is striving hard to explore all possible options to cater to the ever-increasing demand for energy. Conventional energy reserves, viz. coal and fossil fuels, have already been exploited to their maximum extent and their collateral impacts of environmental pollution and global warming leading to climate change. It is indeed essential to scout around various viable sources of renewable energy, such as extracting