RNi8Si3 (R=Gd,Tb): Novel ternary ordered derivatives of the BaCd11 type

The titlecompoundshavebeensynthesizedandcharacterizedbothfromthestructuralandmagnetic point ofview.BothcrystallizeinanewmonoclinicstructurestrictlyrelatedtothetetragonalBaCd11 type. The structurewassolvedbymeansofX-raysingle-crystaltechniquesforGdNi8Si3 and confirmed for TbNi8Si3 on powderdata;thecorrespondinglatticeparameters(obtainedfromGuinierpowderpatterns) are a¼6.3259(2), b¼13.7245(5), c¼7.4949(3)Å, ?¼113.522(3)°, Vcell¼596.64(3)Å3 and a¼6.3200(2), b¼13.6987(4), c¼7.4923(2)Å, ?¼113.494(2)°, Vcell¼594.88(2) Å3. Thesymmetryrelationshipbetween the tI48-I41/amd BaCd11 aristotypeandthenewordered mS48-C2/c GdNi8Si3 derivativeisdescribed via the Bärnighausenformalismwithinthegrouptheory. The largeGd-Gd (Tb-Tb) distances,mediatedviaNi-Si network,likelyleadtoweakmagneticin- teractions.Low-field magnetization vs temperaturemeasurementsindicateweakand field-sensitive antiferromagnetic groundstate,withorderingtemperaturesof3KinGdNi8Si3 and about2-3 K in TbNi8Si3. Ontheotherhand,theisothermal field-dependent magnetizationdatashowthepresenceof competing interactionsinbothcompounds,witha field-induced ferromagneticbehaviorforGdNi8Si3 and aferrimagnetic-likebehaviorinTbNi8Si3 at theorderingtemperatureTC/N of about(orslightly higher than)3K.Themagnetocaloriceffect,quantified intermsofisothermalmagneticentropychange ?Sm, hasthemaximumvaluesof -19.8J(kgK)1 (at 4Kfor140kOe field change)and 12.1J(kgK)1 (at 12Kfor140kOe field change)inGdNi8Si3 and TbNi8Si3, respectively