TriangularPrism[W_, L_, H_, MAT_: DiagonalMatrix[{1, 1, 1}]] :=
Prism[MAT.# & /@ {{0, W*Sqrt[3]/3, 0}, {-W*1/2, -W*Sqrt[3]/6, 0}, {W*1/2, -W*Sqrt[3]/6, 0}, {0, W*Sqrt[3]/3, L + H}, {-W*1/2, -W*Sqrt[3]/6, L + H}, {W*1/2, -W*Sqrt[3]/6, L + H}}];
CircularSubstrate[R_, H_] := ImplicitRegion[x^2 + y^2 <= R^2 && 0 <= z <= H, {x, y, z}];
CompoundStructure[A_, B_] := RegionUnion[A, B]

{L, W, R, H, GridNo, EignNo} = {0.8, 0.7, 4, 1/2, 0.01, 17};

LIxx = FullSimplify[({{[Lambda] + 2 [Mu], 0, 0}, {0, [Mu], 0}, {0,
  0, [Mu]}})];
LIxy = FullSimplify[({{0, [Lambda], 0}, {[Mu], 0, 0}, {0, 0, 0}})];
LIxz = FullSimplify[({{0, 0, [Lambda]}, {0, 0, 0}, {[Mu], 0, 0}})];
LIyx = FullSimplify[({{0, [Mu], 0}, {[Lambda], 0, 0}, {0, 0, 0}})];
LIyy = FullSimplify[({{[Mu], 0, 0}, {0, [Lambda] + 2 [Mu], 0}, {0,
  0, [Mu]}})];
LIyz = FullSimplify[({{0, 0, 0}, {0, 0, [Lambda]}, {0, [Mu], 0}})];
LIzx = FullSimplify[({{0, 0, [Mu]}, {0, 0, 0}, {[Lambda], 0, 0}})];
LIzy = FullSimplify[({{0, 0, 0}, {0, 0, [Mu]}, {0, [Lambda], 0}})];
LIzz = FullSimplify[({{[Mu], 0, 0}, {0, [Mu], 0}, {0,
  0, [Lambda] + 2 [Mu]}})];
{{LIxx, LIxy, LIxz}, {LIyx, LIyy, LIyz}, {LIzx, LIzy, LIzz}} // MatrixForm
PhononEquation3D = {
Inactive[Div][(LIxx.Inactive[Grad][u[x, y, z], {x, y, z}]), {x, y, z}]+
Inactive[Div][(LIxy.Inactive[Grad][v[x, y, z], {x, y, z}]), {x, y, z}]+
Inactive[Div][(LIxz.Inactive[Grad][w[x, y, z], {x, y, z}]), {x, y, z}],

Inactive[Div][(LIyy.Inactive[Grad][v[x, y, z], {x, y, z}]), {x, y, z}]+
Inactive[Div][(LIyx.Inactive[Grad][u[x, y, z], {x, y, z}]), {x, y, z}]+
Inactive[Div][(LIyz.Inactive[Grad][w[x, y, z], {x, y, z}]), {x, y, z}],

Inactive[Div][(LIzz.Inactive[Grad][w[x, y, z], {x, y, z}]), {x, y, z}]+
Inactive[Div][(LIzx.Inactive[Grad][u[x, y, z], {x, y, z}]), {x, y, z}]+
Inactive[Div][(LIzy.Inactive[Grad][v[x, y, z], {x, y, z}]), {x, y, z}]} /. {[Lambda] -> -85*10^(3)/((2*[Pi])^2*3512), [Mu] -> -536*10^(3)/((2*[Pi])^2*3512)};
Cavity = CompoundStructure[TriangularPrism[W, L, H],CircularSubstrate[R, H]];
Pillar = TriangularPrism[W, L, H];

Subscript[[CapitalGamma], CircularSubstrateClampedEdge] =DirichletCondition[{u[x, y, z] == 0, v[x, y, z] == 0, w[x, y, z] == 0}, x^2 + y^2 == R^2 && 0 <= z <= H];
Subscript[[CapitalGamma], SubstrateClampedBase] =DirichletCondition[{u[x, y, z] == 0, v[x, y, z] == 0, w[x, y, z] == 0}, z == 0];
Subscript[[CapitalGamma], PrismClampedBase] = DirichletCondition[{u[x, y, z] == 0, v[x, y, z] == 0, w[x, y, z] == 0}, z == 1/2];

mparams = {"Eigensystem" -> {"FEAST","Interval" -> {(50 - 0.145)^2, (50 + 0.145)^2}, "SubspaceSize" -> 50}, "SpatialDiscretization" -> {"FiniteElement", {"MeshOptions" -> {"MaxCellMeasure" -> GridNO}}}};

{[Omega], f} =  NDEigensystem[{PhononEquation3D, Subscript[[CapitalGamma],    SubstrateClampedBase]}, {u, v, w}, {x, y, z} [Element]Cavity, EignNo,Method -> mparams]