We report here a new class of ammonium metal-formate frameworks of[NH_(2)NH_(3)][M(HCOO)_(3)](M=Mn^(2+),Zn^(2+),Co^(2+)and Mg^(2+))incorporating hydrazinium as the cationic template and component.The perovskite Mn and...We report here a new class of ammonium metal-formate frameworks of[NH_(2)NH_(3)][M(HCOO)_(3)](M=Mn^(2+),Zn^(2+),Co^(2+)and Mg^(2+))incorporating hydrazinium as the cationic template and component.The perovskite Mn and Zn members possess anionic 4^(12)·6^(3)metal-formate frameworks with cubic cavities occupied by the NH_(2)NH_(3)^(+)cations,while the Co and Mg members have chiral 49·66 metal-formate frameworks,with chiral hexagonal channels accommodating NH_(2)NH_(3)+cations.On heating,the Mn and Zn members undergo phase transitions around 350 K.The structures change from low temperature(LT)polar phases in Pna2_(1)to high temperature(HT)non-polar phases in Pnma,due to the thermally activated librational movement of the NH_(2)end of the NH_(2)NH_(3)^(+)in the cavity and significant framework regulation.The Co and Mg members in LT belong to non-polar P2_(1)2_(1)2_(1),are probably antiferroelectric,and they show phase transitions at 380 K(Co)and 348 K(Mg),and the structures change to polar HT phases in P6_(3),triggered by the order-disorder transition of the cation from one unique orientation in LT to three of trigonally-disorder state in HT.Accompanying the phase transitions,which are ferro-to para-electric for Mn and Zn members while antiferro-to ferro-electric for Co and Mg,prominent anisotropic thermal expansions including negative ones,and dielectric anomalies,are observed.The spontaneous polarization values are estimated at 3.58(Mn,110 K),3.48(Zn,110 K),2.61(Co,405 K)and 3.44(Mg,400 K)μC cm^(-2),respectively,based on the positive and negative charge separations in the polar structures.The structure-property relevance is established based on the order-disorder transitions of NH2NH3+and the conformity and adaptability of the metal-formate frameworks to match such order-disorder alternations.The Mn and Co members show spin-canted antiferromagnetic long-range-ordering,with Néel temperatures of 7.9 K and 13.9 K,respectively.Therefore,the two members show coexistence of electric and magnetic orderings in the low temperature region,and they are possible molecule-based multiferroics.展开更多
基金supported by the NSFC(grants 21171010,21290170,21290171)the National Basic Research Program of China(grant 2009CB929403).
文摘We report here a new class of ammonium metal-formate frameworks of[NH_(2)NH_(3)][M(HCOO)_(3)](M=Mn^(2+),Zn^(2+),Co^(2+)and Mg^(2+))incorporating hydrazinium as the cationic template and component.The perovskite Mn and Zn members possess anionic 4^(12)·6^(3)metal-formate frameworks with cubic cavities occupied by the NH_(2)NH_(3)^(+)cations,while the Co and Mg members have chiral 49·66 metal-formate frameworks,with chiral hexagonal channels accommodating NH_(2)NH_(3)+cations.On heating,the Mn and Zn members undergo phase transitions around 350 K.The structures change from low temperature(LT)polar phases in Pna2_(1)to high temperature(HT)non-polar phases in Pnma,due to the thermally activated librational movement of the NH_(2)end of the NH_(2)NH_(3)^(+)in the cavity and significant framework regulation.The Co and Mg members in LT belong to non-polar P2_(1)2_(1)2_(1),are probably antiferroelectric,and they show phase transitions at 380 K(Co)and 348 K(Mg),and the structures change to polar HT phases in P6_(3),triggered by the order-disorder transition of the cation from one unique orientation in LT to three of trigonally-disorder state in HT.Accompanying the phase transitions,which are ferro-to para-electric for Mn and Zn members while antiferro-to ferro-electric for Co and Mg,prominent anisotropic thermal expansions including negative ones,and dielectric anomalies,are observed.The spontaneous polarization values are estimated at 3.58(Mn,110 K),3.48(Zn,110 K),2.61(Co,405 K)and 3.44(Mg,400 K)μC cm^(-2),respectively,based on the positive and negative charge separations in the polar structures.The structure-property relevance is established based on the order-disorder transitions of NH2NH3+and the conformity and adaptability of the metal-formate frameworks to match such order-disorder alternations.The Mn and Co members show spin-canted antiferromagnetic long-range-ordering,with Néel temperatures of 7.9 K and 13.9 K,respectively.Therefore,the two members show coexistence of electric and magnetic orderings in the low temperature region,and they are possible molecule-based multiferroics.
